Why cause to pre-cum?

it happens to me a lot when i am thinking about about sexual things, and when i am on the phone with a girl. the pre-cum just starts to explode why cause it to happen? is it because i am to excited of thinking what would happen next? i don't that's the real problem though, because i am often relaxed and calm about the situation i guess its all dude got the same problem as me? is this normal?