There are as many nights as days, and the one is just as long as the other in the year's course. Even a happy life cannot be without a measure of darkness, and the word 'happy' would lose its meaning if it were not balanced by sadness. -- Carl JungThere seems to be a popular opinion that being happy all the time would be boring (and hence unhappy). Such a belief, I think, stems from poor intuition about emotion. Plus, there's a logical fallacy there.
First: is there any difference between a chronically depressed person and you (assuming you're not the former)? His relative mood is probably much like yours: it elevates a bit when good fortune strikes, and depresses when things go wrong. If happiness were defined by contrasts, it wouldn't be meaningful to say that you're happier than he is.
But the huge size of the antidepressant market is good evidence that this isn't the case. So why is it hard to believe that there exist people for whom your situation is analogous? Would you prefer to be miserable because everything else is boring?
Second: to say that being happy all the time would be unhappy implies that there are two different usages of "happiness" here. We can't logically be simultaneously happy and unhappy if they are to refer to the same dimension, but we could conventionally say such a thing (and sometimes do, colloquially) if they mean slightly different things. But if so, and it's the latter meaning that we're emphasizing and trying to optimize ("...then you would be unhappy"), then we can meaningfully ask: what if you were consistently happy in that sense? It logically precludes the possibility of you being anything other than happy.
Another take is that being unhappy all the time leads to unhappiness. In this way, there is only one state we're discussing. But now we're making a behavioral claim about how people must react (as opposed to a philosophical claim about how mental states are relative), and plenty of evidence from positive psychology indicates that we're pretty wrong about the upper reaches of mental ability.
Look at it this way: is "tall" a relative term? Yes, but only in relation to other people. You're not "tall" because you're taller than you were a few years ago. If you vary in height between 4' and 4'8", you're still never "tall." And if you're 8' in a world full of 8'ers, you may not get the distinction of being "tall," but you can still damn well dunk on an 8' rim.