I think there's a bit of selection bias here. Often times the only euphoria cases you really hear of are the ones that come along with some sort of detriment, leading the patient or people around them to seek help. I have a gut theory that that chronic elation is just as prominant as chronic depression, but since it rarely effects the person adversely few seek help for it.
Throughout my life I've been for the most part overly happy. It's just something myself, my family, and those close to me have noticed. There are issues with it, but they tend to be minor and things that I wouldn't really seek help for:
-being happy in inappropriate contexts (people tend to not resonate with you when you're happy at a funeral).
-much higher tolerance with ambiguity. I'm OK with being lost somewhere, which tends to annoy those around me as I tend to wander towards a destination rather than taking an optimal route.
-and an exageration of smile wrinkles on my face.
The negative impact to my life is quite low. Since I'd never seek help for it, it would never really be recorded anywhere. I suspect there are plenty of people with a chronic elation that are in the same boat - there's no onus to address it as an issue. I have no idea what makes me tick differently, but I suspect with a larger understanding of human biology we'll eventually be able to isolate these things and find an optimal amount of happy for humans to have as a baseline.