Misery loves company? Hah, I think the closer answer is that they are seeking social conformity. Or, in another way to think of it is people expect people to get married in life. Like, 'when are you getting a car', 'when are you getting a job', 'when are you getting a house'.
I also think, that people like to complain a lot. I complain a lot about my relationship but don't mean half of it, and the things I do mean are compensated by the good qualities of being attached.