Why does a person get obsessive, Specially towards those who don't accept his love or attraction towards them?
Self esteem has such an impact on human life that he wants to preserve his esteem in any way possible. Then why does one get obsessive towards those who have broken the trust he put into them? Why does he try to reason with those who have told him that they don't love him?
Where does his drive to consolidate his self esteem go in such situations?
Another component, which relies less on situations from one's past, are the effects of perceived scarcity. A paradigm in behavioral psychology which has become almost universally accepted is that the more scarce and hard to come by something is (be it a certain model of car, or the affection of another individual), the more valuable we perceive it to be. This is why a common sales tactic is to tell a potential customer that the item they are prospecting is one of the last in stock.
If you expand this principle into human interaction, its easy to understand why not reciprocating someone's interest can often lead them to double down and perhaps even desire attention and affection more than they originally had.
There are in many cases certain attachments that are natural to the human condition. It is normal and natural to want your immediate family and sometimes extended family to love you. As humans we form long lasting relationships where we hope that our spouses and friends will become family as well and love us. However this is not always possible. Some fathers, mothers, siblings, lovers and friends cannot show or do not have the natural love or care. A normal person grieves for the loss of a close relationship. Part of the process of grieving typically involves denial and bargaining.
Sometimes this lack of love is caused by a legitimate mental illness like Borderline personality disorder or versions of Bipolar. In some cases of BPD the loved one may swing between I love you and I absolutely hate you. The smallest or largest of offenses can cause the switch and sometimes it lasts for long periods of times Hours, Months or Years. Its difficult to deal with. You always hold out hope that the intensity of the relationship will swing back in your favor.
People who grow up with mentally ill parents or experience such relationship with those they are close too have learned behaviors of dealing with rejection in a way that is not healthy for their self-esteem. It can result in any number of illnesses including pathological and nonpathological OCD, Dependant Personality Disorder or Codependency as you some what describe.