I don't think a person who is single and childless has a lesser quality of life per se than a married person or a parent. There are plenty of couples and parents who are unhappy and unfulfilled.
But for me, the first time I held my daughter (my oldest) in my arms right after she was born, I have never felt more "needed" in all my life. I was holding this tiny, frail little being for which I was directly responsible. I was overwhelmed by that sense of "neededness" (sorry that I don't have a better term to describe it). Now that she's nearly a teenager, I still get choked up with love for her when I look in her eyes or when I think about her (and my other 2 kids). They mean more to me than my own life. I'd rather suffer pain, illness, or injury personally than see them suffer in such a way. To me, there's no greater sound in the world than to hear your kids laughing and then there's no worse sound in the world than to hear your kids crying in pain.
Getting married and having children changes your life in a way that nothing else can. You give up a lot when you do those things (complete independence, e.g.) but life becomes completely different, but in a good way (at least, I choose to see it that way... most of the time... ha ha).