I Do Not Understand

I don't understand some things right now.  Yesterday, I went out with some friends and we all had a good time for the most part.  It seems like the topic of friends seems to always be the cause of my deep thinking and saddened moods.  I went out with some friends again tonight; we walked all the way to some party and then I just had to walk back because I was not feeling it.  I like them, they are nice people and most of them talk to me and try to include me in things and stuff, but then it seems like when I'm there with them I am just invisible.  I know this is not true but that is what it seems like, there are some of them who are more friendly towards me than others and I appreciate it, but the whole thing just makes me sad.  Even if I wanted to take a break from them and hang out with some other friends or something I would not be able to.  I have a couple other friends that don't really hang out with them, but it is just frustrating.  I don't understand why I have such problems making friends.
For example, I met this guy last year at this out of state reception thing.  I see him all over the place, and he seems to know everybody, however he is just two-faced.  Sometimes I will see him and say hi and he'll respond, but then other times when I try to say hello to him he just ignores me and looks at me like I'm stupid.
I consider myself to be a nice and friendly person, especially towards people I do not know, however I still can't seem to just make friends with someone.  Also, like all of my friends are girls, I don't know why.  It's really irritating I can't ever make any guy friends, my roommate is the one guy friend that I've made since all the time I've been going to school here in Flagstaff, it's really pretty sad.