Yea, I'd assume the chronically unhappy/depressed are probably less likely to find a long-term partner than average.
It would potentially be also interesting to control for sexual activity? Presumably most married couples were sexually active together at some point, which I'd assume generally increases happiness.
It could also be that people who are generally happier tend to be those to get/stay married.
I also wonder if there is some sunk cost psychology involved when surveying those later in life.