As a psychologist, I could write a few books on this one. Not having an answer is pretty common. Inuman has a point, but sounds a little bit blaming, making assumptions without evidence of why an individual doesn't have a partner, that there is something bad about that individual. I am 52, very articulate, kind, never been violent with a partner, steadily employed, etc. I have had partners for two years or less. Now you can look for reasons why I as an individual have problems in becoming partnered or you can look at culture wide patterns of attachment in heterosexuals and gay men. 90% of the general population marries at some point. In 2007, there were 2,205,000 marriages. How many gay marriages were there? My point is that our culture supports heterosexual unions and actively discourages gay unions. In that context, the better question is to those who are coupled: how the hell do you manage to stay together in this culture? Most of us have very little support for our relationships. My nephew, a very fine man, at 27, has been married to a very fine young woman for four years. He and his wife just had twins. He and his wife have much needed support from two extended families. Financial support, logistical support, emotional support to the extent possible in these families. Most of us are not part of the breeding system and are, at best, peripheral to our families.
In addition to the lack of general support from families of origin, either indifference, active sabotage, or conditional support, gay culture supplies very little support for partners. Historically, couples were very hidden due to active homophobia (consider the 20th century) and public campaigns to ensure that homosexuals were discharged from jobs. For most of the 20th century, gay culture was about survival and existed only as entertainment for brief moments. There have been very few institutions which support gay people within or outside the gay community in having partners. Predominately gay churches have been the institution which has supported gay unions and there are very few such chuches and they are still not very robust.
Now, our cultural and historical context explains, in part, why the rate of unions is much lower among gay men as compared to heterosexual men, but why do certain gay individuals pair up and others don't. I think then you can look at attachment patterns and styles amongst gay people as a whole and as individuals. Let's just say that growing up isolated in our heterosexual families of origin (for the vast majority of us) has not prepared us in any way to couple with another man. I could go into great detail about this, but that could end up being another book.
I look at myself and my personal history and know that I was lucky to survive growing up in the family I did. My brother, who was heterosexual, did not survive.
My short answer to that question is, "You know, I ask that question myself. Do you have any ideas?"