I think it's when you think about that person a lot and can't seem to get them off your mind. You're really happy when you are together and feel like it's almost a dream. And as soon as you aren't with them you begin to really miss them. That's what it's like for me anyway.
I think when I look at them or if I have the opportunity to talk to them, my heart would be racing. And sometimes the person I like appears in my dream lol