As a single dude who enjoys being single, I have to say that relationships are simply not for everyone. It really depends on what a person needs/wants at any given time in her/his life. At this point in my life, I am not looking for a relationship, nor do I want one any time soon. We all know relationships take a lot of work (and I don't mean that in the "ugh, groan" sense), and I think it's important to know who YOU are and what you are WILLING to give and not give in a relationship. So many people like the IDEA of being in a relationship - that is, they romanticize (even fetishize) the idea of relationships - that when it comes to actually being in a real relationship with real feelings in the real world, some people are just not cut out for it.
I have been in relationships before, and I can say with all certainty that I enjoy being single a LOT more than I enjoy relationships. I do take offense to those who are in relationships that think because they are in a fantastic relationship, they are somehow better than those who aren't, and that single people are to be pitied because they are obviously missing something they deem "necessary" and "important." The fact is that some people choose to be single...and there is nothing wrong with this. Different strokes for different folks.
Am I closed to having a relationship? No. But it's not my goal, nor is it something that pains me. If it happens, it happens. If not, hey - I'm having a good time!