Why our heart is beating too much when we are talking to our crush?

It's a physiological reaction to a psychological issue. Because unconciously your brain tells your body to freak out. You don't ACTIVELY tell your body that, but your brain does for you. It's a variation of the fight or flight reflex. if you wanted an actual answer =)

Because you are nervous, I think i read somewhere that this reaction is related with the fight or flight instinct.

Because we get scared they'll reject.