When we long for someone, we do indulge in obsessive thinking. Lovers are infamous for making mountains out of a molehill. Even the most trivial of the things can make you edgy.
Jealousy, the green-eyed monster, raises its ugly head, even if you see your love interest talking casually with another person of the opposite sex. Many people become extremely possessive, and act unreasonably. Desire, fear of rejection, and a host of other emotions, make you behave the way you do.