My theory about why I don't have a girlfriend boils down to: girls don't like what I have to offer.
I'm the kind of person that isn't necessarily immediately likable. I love to think, and that usually manifests itself into me challenging people's ideas even if its just in a "devil's advocate" sense. I have plenty of meaningful friendships, but it takes time to understand where I am coming from.
Generally speaking, since girls have control of the dating market, I'm not nearly as appealing as somebody who seems to agree with their values, even if the guy is speaking a load of shit just to get laid. Since I value sex far less than I value a meaningful relationship, I'm waiting to meet a girl who appreciates my unique qualities.
Statistically it makes sense that I'm single. As a 24 year old who is no longer bunched together with a substantial number of peers like in high school or when I lived in a college dorm, I don't meet new people all that often. Most (attractive) girls I meet are serial daters, so they require that perfect storm moment when they are looking for something their current boyfriend isn't offering. Finding a girl that is single and looking is rare enough, and then finding a girl who appreciates my sarcastic sense of humor and liberally minded ways is even tougher.
My past relationships have all been mentally unhealthy. Part of that is that since I'm bi-polar and suffer from anxiety, the girls I most relate with have similar issues. This has sort of poisoned me to the opposite sex, and given me a skewed view of how women are.
It used to really bother me that girls generally don't want a relationship with me, but I've come to terms with the fact that I don't need a partner until I find somebody who I'm compatible with. In many ways it's nice to live alone and watch what I want, play video games when I want, and go to bed whenever and wherever I please.
I've been single for the last year, and I see it as a way to more fully understand myself without being distracted by somebody else's desires. It's important to fully develop independence before any co-dependence. My mom tells me that since she got married at my age, there are some things that she never learned. She's never handled personal finances or negotiated with a car dealer, and she didn't develop self-confidence until she became a mother in her 30s.
My friend recently remarked that she doesn't feel comfortable without sleeping next to her boyfriend - a relationship she's had since high school - so at least I get the benefit of knowing that the source of my happiness lies in me, and not a significant other.