Why do you get butterflies in your stomach all the time?

That's just a saying. You don't really get butterflies in your stomach. It only means that you feel nervous and quivery.

If you're around a guy, it normally means you got this HUGE crush on him.

It's when you have an adreniline rush and it makes something happen inside you which gives you butterflies. it usually occurs when you are excited, nervous, or anxious.