How has Christianity become so legalistic?

I'm a Christian and I get many complaints from friends that Christianity is legalistic & sort of "the man" in a sence. It's really interesting to hear this because, when you read Jesus' stories, he was the rebel of rebels. I mean, talk about a rebel. Jesus was the emo rebel kid of His day times 1000. He hung out with prostitues, Samaritans (the hated race of his day), tax collectors (the irs of his day), the unclean and practically invented women's rights. How is it that Christianity has become all about legalism when, if you look back, Jesus was the greatest rebel of all time?