What is this thing even called?

there's this guy i like him i guess and whenever he's around i instantly feel good and my heart beats faster and eyes sparkle. we never got close because we ha a very weird relationship whenever he started something it scared me and i acted stupid and now he's leaving forever and i miss him already if there's even a slight mention of him i almost start crying. how can i feel this way and be so into him if we're basically no ones to each other?