Can someone explain what is wrong with me?

Okay, so this has happened me to me three times already. Someone confesses to me. I follow my heart and tell them I like them back. So, at first I'm completely infatuated with them. I think about them all the time. I get those crazy butterflies and that nervousness around them. I want to be next to them.. After a few days, it just stops. I just look at them and feel nothing. I don't want to be anything but friends with them... Can someone explain what this is? I don't know. I feel like I'm incapable of liking anyone for a long time now..