Why am i unpopular?

I have a few friends but not a whole lot. Whenever I'm hanging out I'm friendly, I say hello to people, and I start conversations. But people are usually rude to me. If I sit there quietly people ignore me, and if I ask questions and start conversations they often ignore me anyway. I'm not mumbling or speaking quietly so that's not the problem. I'm not ugly so that can't be the problem either. For a while I thought people must think I'm conceited but I don't act like I'm conceited, meaning I don't talk about how good looking I am and I talk to people who are less attractive than me. Generally women are interested in me at parties and stuff and I try to be a straight arrow, not stepping on anyone's toes. Yet I rarely if ever get invited to parties that aren't being held at my house, and people don't call me to hang out. I try my hardest not to be overbearing or wimpy. So what am I doing wrong? Why don't people call me?