Why do people take advantage of others?

I started out as a nice guy when joining a church group. However, this church did not turn out that nice. The members expected me to go wild and crazy at sporting events, sing karaoke when going out to eat, and even ask me questions such as personal ones involving going to the bathroom or talking to a girl. I ignored them, and I even refused to listen to them, but they still claim me as an easy target. Why? Also, like I said, I started out as a nice person to them.