Specifically, I don't think there is any evidence that happiness comes from having "solved" problems in your life. It is well-known that winning the lottery doesn't change happiness -- you get happier for a short period, but then people seem to re-normalize to the new wealth level. Other changes in life circumstances seem similar.
Happiness as a transitory emotion certainly exists. But is happiness as a reportable statistic (like weight or income) meaningful? I think the null hypothesis should be "no". It isn't clear what we are measuring, or what it means in terms of our lives. If I report that I am happy, there is some notion that it is durable and meaningful, but three hours later, I may think about my relationship with my estranged kid and be feeling blue.
I don't think "happiness" has any meaning beyond "the emotions I feel this moment", which are inevitably a reflection of "what I have been thinking about in preceding moments". If I am working on an engineering problem and come to a satisfying solution, am I "happier" than I will be when thinking of my ex-wife?
As local "emotional weather", it's meaningful. As a reportable statistic, it's garbage.