One thing is that for every couple made up of two people who are absolutely happy and in love, there a whole lot more who are not. Or else, there's always the ones who feel bad for single people while the single person is thinking, "Thank God I didn't find myself married to someone like she/he is married to!!" (lol)
I have no doubt that there are some couples who are truly happy (both of the individuals in them - not just one happy one and one the other THINKS is happy, sometimes only because the unhappy person has't yet left), but a whole lot of people settle for a "B-minus", "C-minus", "D-minus", or even out-and-out "E" relationship because they don't believe there's such a thing as an "A" or "A-plus" relationship.
One problem can be that a "B-minus" relationship can seem quite happy, and maybe that has to be good enough for a lot of people. The trouble is, however, that it may not take much to knock a "B-minus" relationship down to a "C-minus"; and from there it's sometimes only a matter of luck if that "C-minus" stays at least that good, or further declines.
People so often (rightfully) are aware that things most often are not perfect in any relationship, but it's easy to blur the fact that "you have to work in any relationship" with "if things really were the way they should be in the relationship you wouldn't have to always work quite that hard at it".
As for singles and how happy they are being single, I suppose it's both simpler and more complicated in a lot of ways. Some singles plain, old, don't want to be in a relationship for one reason or another. Some are selective, and only want to be in a relationship if it can be an "A-plus" one (even if day-to-day stuff requires some compromise and work). Singles are individuals, just as couples are individual-couples made up of individuals. Some singles can't, and won't, ever be happy unless they're in a relationship; so whether they settle for a "C-minus" one or a "D-minus" one can depend on how important having an "A-plus" (or at least "A-minus") one is, or isn't for them.
So, when it comes to who is happier (singles or couples), I just think it depends on so many things other than just who is single and who is part of a couple.
Really, what every person needs to do is ask himself/herself whether he/she is truly happy with his single-/couple- status, but also be brutally honest with himself/herself over that. Too often, perhaps, too many people (whether single or in a couple) choose to lie to themselves because while lying to oneself doesn't do much for one's true happiness, it's sometimes easier than facing whatever truth there is to face.
Photo: ME Whelan