Limerence is a state of overwhelming and unexpected longing for emotional reciprocity from another human being. It differs from a crush in its intensity: it is an emotional roller coaster that fluctuates between euphoria and despair.
For the most part, people are serial limers and get caught up chasing the same dopamine spike that is felt in the beginning stages of love. The experience of limerence is immemorial, but the term is relatively new and was coined by Dorothy Tennov in 1979.