Conventional wisdom generally attributes this difference to ethereal things like emotional support and social stress levels.
I think it's much more prosaic.
I think it's the cumulative effects of having someone else around to help deal with all the little aches and pains that you can't normally can't reach, like zits on your back.
That, and having someone around who will say, "It's been two weeks, would you go to the damn doctor already? And take your damn pills!"