Certainly the adversary that has caused the most personal harm to Spider-Man than any other criminal, Norman Osborn's hatred began early in life. His father Amberson Osborn, was an inventor in his own right and an industrialist. At some point, Amberson lost control of the company and the family wealth. (The Osborn's seem to be an "old money" family, as Norman once explains that an ancestor of his was accused of being a robber baron.) Amberson began to drink and took his anger out on his wife and child. Already showing signs of antisocial personality disorder, young Norman killed the family dog, justifying it as "one less mouth to feed." Whether or not he was really looking out for the family finances or if he just killed the dog out of anger at seeing Amberson beat his mother is uncertain.
Norman took another step towards psychological trouble when Amberson tried to cure his son's fear of the dark. Amberson tried to remedy this by locking Norman alone in the house overnight during a thunderstorm. In his terror, Norman imagined a green monster was in the dark, waiting to eat him. The creature would only get him in the brief flashes on light from the lightening, so Norman prayed for the dark to stay forever. He decided that the dark was always better than the light. Years later, he also used his fear of the monster as inspiration for his criminal persona.
Norman took it on himself to restore the Osborn name and fortune. He studied chemistry in college, totally focused on his school work. Somehow he met a woman named Emily, who could bring the brooding young man out of his shell. She would tell him not to sulk so much and that she married him because she loved him, not for his family name. Norman loved having a woman fuss over him and he couldn't take it after Emily died. He raised their son, Harry, alone. The death of Emily remained painful to Norman for years, and he would strike out at Harry for even mentioning her name.
Osborn did eventually manage to re-create the family wealth, with help from his university professor, Dr Mendel Stromm. They went into business together but Norman soon had him framed for embezzlement, giving Norman full control of the company. After Dr. Stromm was fired and sent to prison, Norman went through his files and found a formula for increasing strength and intelligence. He re-created the formula, which exploded in his face, covering him in the substance. He recuperated in the hospital and found that he had indeed increased his strength and intelligence...but couldn't admit that the formula had also ruined his sanity.
Norman set about controlling the New York mafia, in a bid for even more money and power. He started out with proxies to do his dirty work, but they were all soon captured by the new vigilante, Spider-Man. Taking the adage, "If you want something done right, do it yourself," to heart, Norman created the Green Goblin costume and set about trying to kill Spider-Man. He hoped that the vigilante's death would make a name for him in the mob, and he would eventually come to control them. Defeat after defeat proved him wrong, even though the Goblin did manage to escape with his secret I.D. intact.
Norman gained his greatest triumph after learning that Peter Parker is Spider-Man and he set about making his life a living hell. Other enhanced criminals threaten Spider-Man's family, but Norman made good. After seducing Parker's girlfriend, Gwen Stacy, she gave birth to twins, Sarah and Gabreil. She refused to give up custody to Norman and he went to her apartment as the Goblin and kidnapped her. Leading Spider-Man to a New York bridge, he killed Gwen in front of her boyfriend. The resulting battle seemed to end with the Green Goblin's death.
Little did anyone know, but the Goblin formula also gave Norman an advanced healing factor and he woke up in the morgue. He faked his death and fled to Paris, where he continued to plot against Parker. The battle of wills continues, with both parties refusing to give in out of spite.
Psychopathology: Bipolar Disorder, Obsessive Compulsive Disorder, Paranoid Personality Disorder and Narcissism (What Doesn't This Guy Have?)
As Dr. Karla Sofen noted, Norman is undoubtedly severely bipolar. (Thunderbolts #113) Norman's signs of this mental illness likely first showing up as a child, with his vivid fears of a monster out to get him. He was still a disturbed young man in college, brooding constantly over the lost family fortune. He likely stayed away from parties in college, seeing them as a waste of time. How he managed to meet, much less marry, a kind woman like Emily is a mystery. Regardless, her death was a major stressor in his life and likely helped send him down the dangerous path that he took.
Stress can be a major factor in bringing about a manic episode in a bipolar person. Norman has shown to be a stress-prone individual, with manic episodes resulting from stressful situations. Some of his most violent acts have been preceded by stress. Gwen Stacy's murder followed Norman losing 13% in his stocks, Harry falling ill from his drug use and Gwen refusing to let Norman take custody of their children. (Amazing Spider-Man #121 and Amazing Spider-Man #512) This episode was so severe, it caused Norman to hallucinate that Spider-Man had come to gloat over his troubles.
In a much later incident, Norman was tricked into going off his medication while director of the Thunderbolts. This caused him to literally crucify the team member Swordsman for attempting to stage a coup against Osborn's leadership. (Thunderbolts #120) In general, Osborn shows very poor impulse control. When a reporter from the Daily Bugle came to interview him on missing employees from his company, he killed her and dumped the body in the reservoir. A further investigation from the Bugle reporters led them to Osborn, with the police in tow. Instead of letting the police look around and give them an interview, Osborn flew off the handle and killed the lot of them, except two Bugle reporters. (The Pulse #4)
Osborn is also an obsessive planner. According to Mac Gargan, aka the Scorpion, Norman has "hundreds of plans and counter plans" to hurt Parker. He tells Peter: "You ever been in his hideout? You ever see those schemes and arrows and boxes he has all over his walls? He's had this one planned out since the minute he found out who you were, kid." (Marvel Knights Spider-Man #9)
While many people suffering from obsessive compulsive disorder obsess over things like dirt or germs, Norman obsesses over enemies, real and perceived. Indeed, Norman believes he has enemies everywhere: Spider-Man, J. Jonah Jameson, members of the mafia, other members of the Thunderbolts, Spider-Man, his doctors, the news media and Spider-Man. Look at his thoughts as Harry lay sick in bed: "I should have realized those--those friends of yours--what they were doing to you! But how could I have known? Our enemies are everywhere...men trying to ruin me...corrupt you, my only son...enemies...surrounding us..." (Amazing Spider-Man #121)
Finally, narcissism is a major part of Norman's psyche. He clearly sees himself as a "cut above." Being rich and famous is a source of great pride for Osborn (he named his freakin company after himself), and he likely is irked by the fact that he is consistently beaten by a underpaid nobody living out of cheap apartments. He made this clear after Gwen's death, asking Spidey "What worth is there in the paltry existence of one useless female? A simpering, pointless girl who never did more than occupy space--while I--" He was cut off by Spider-Man, who resumed beating his enemy to a pulp. (Amazing Spider-Man #122)
Norman Osborn: Assassin
Osborn's paranoia and obsession with Spider-Man gives him some similarities to assassins, like John Hinckley. (Hinckley tried to kill President Reagan in 1981, to win the affection of Jodie Foster.) The faulty logic of the thinking is clear. Just as Jodie Foster would want nothing to do with Hinckley had he killed the president, the mob wouldn't care if the Green Goblin killed Spider-Man. Norman is a classic "loner." He never re-married after Emily died, he had few friends, didn't do much outside of obsess over Osborn Industries (and Spider-Man) and always had weapons nearby. He had plenty of Goblin hideouts across the city, for him to make quick changes if necessary.
Norman Osborn's main goal in life is the accumulation and maintaining of power. Whether it comes in the form of wealth and fame from Osborn Industries, as the leader of the Thunderbolts or as the Director of H.A.M.M.E.R., power is his first and last preoccupation. Woe to anyone that gets in his way: Spider-Man, Swordsman, Iron Man and others. Norman's psychosis goes back to his traumatic childhood and his drunken, abusive father, as well as his genetics. Heavy medications may bring some of his symptoms under control, but he must keep up with them daily. To do this, he would have to think about something besides power and crushing his enemies.