Why are guys so open and social with girls?

So clearly females don't know too much about what goes on in guys' heads but it confuses me. Why are they so open to socialize with girls they don't even know? I was at six flags yesterday with some friends and we rode front row on goliath with a random guy who was probably my age. After we got off he asked for my name and cell number, told me I was hot, and asked me to join his group. I couldn't leave my friends so we just all got together. It's weird though because this happens anywhere, like the mall, the movies or any other adolescent-filled area. Why do guys just randomly talk to girls?