You get really anxious when you're not talking to that specific person, like, all day. You feel extremely protective of their name and everything they do is amazing. You think they're flawlessly beautiful even if they're just average. And so on :)
See, you just... Know. You always get butterflies when you seem them, even if you're p!ssed off at them. You feel comfortable yet nervous while around them. And they can always put a smile on your face.