Why do Christians try to save people?

Lately I have been going through a promiscuous phase. I had a horrible break with my last boyfriend. What is upsetting is my best friend who is a Christian keeps telling to go to church. I agree she is being positive by trying to encourage and help me, but I find it condescending when she tells me my behavior is immoral and against Jesus's teaching. Who the hell is she to have the nerve to bust out her bible to me and preach to me? Yes, she literally takes out her bible and find sections in the bible that say pre-marital is wrong. I agree her intentions are well, but I feel she is condescending. How should I do tell her to back off?