Obsessive love, as the name implies, is an unhealthy state of mind a person can enter in which they develop a powerful obsession with a person they feel strongly attracted to. This condition naturally has a disruptive effect on the sufferer's ability to think clearly, not unlike the way alcohol does-some people refer to sufferers as being "love drunk." Some minor characters the six main protagonists on Friends encountered appeared to be sufferers of this mental state, all of whom are listed below.
Erika Ford appears in "The One After the Superbowl, Part 1". She is an insane woman who began stalking Joey, believing he was in fact Dr. Drake Ramoray (the fictional character he portrayed on Days Of Our Lives. She could not understand that it was just a TV show and that Drake wasn't even a real person. Ross got rid of her by suggesting that Joey was in fact Drake's "evil twin" Hans Ramoray. The friends played along and suggested that Erika go to Salem to find the "real" Drake.
Malcolm was a man who began stalking Ursula after a relationship with her ended with her dumping him, and ended up stalking Phoebe by accident after mistaking her for Ursula, to his own embarrassment. Malcolm seemed to be aware of the fact that his obsession with Ursula was a bad thing and seemed to hate himself for his inability to stop thinking about her, calling himself "pathetic." Despite this level of self-awareness, his obsession seemed to be making him "love drunk", causing him to buy a pair of night-vision goggles which enabled him to spy on her and keep a journal where he could keep record of her every move. Even after entering a relationship with a sympathetic Phoebe, who understood that "this is what (Ursula) does to men," he could not stop thinking about or stalking Ursula, to his own and Phoebe's chagrin. However, he apparently did eventually get over his obsession with Ursula and stop stalking her, since he was never seen again in the series after the episode he appeared in, while she continued to make further appearances.
Dr. Harad is the obstetrician who delivered Frank & Alice's triplets caused Phoebe to initially take a dislike to him by relentlessly, and sometimes completely spontaneously, talking about the Happy Days character Arthur Fonzarelli, who he claimed to "love". He most likely had an admiration towards the Fonz, which at times seemed to be making him "love drunk", such as when he felt the need to tell Rachel and Phoebe that "I'm not Fonzie," and later when he seemed to be upset when Rachel claimed to be a fan of Mork from the sitcom Mork and Mindy ("Fonzie met Mork. Mork froze Fonzie.") calming down only when Rachel convinced him that "Fonzie was already cool. So he wasn't hurt, right?" Despite his Fonz obsession, Harad ultimately proved himself to be a highly competent doctor who successfully delivered all three of the triplets.
Michelle was a woman Ross met in the coffee shop after overhearing her crying and pouring out her emotions to a female friend after her boyfriend broke up with her. With crassness and thoughtlessness that left Chandler astonished, Ross then hit on her, causing her to develop an obsession with him which seemed to be making her "love drunk", given the bizarre, insane comments she kept making. Quickly realizing he made a mistake, Ross got rid of her by convincing her to leave and, when she pointed out that he didn't know her phone number, he promised to merely guess what it was. It is safe to assume that she did however get over her obsession with him, since she was never seen again in the series after the episode she appeared in and apparently never bothered Ross again.