I asked my Japanese wife for her opinion on this matter and she thinks the reason Japanese in general are more content is because of the lack of drive created by class inequality. Generally speaking, Japanese people are fairly well taken care of by the social systems and there is not a lot of flaunted wealth unlike in the west. Most Japanese probably see no need to strive and work too hard because their life is okay by most standards. Food is great, healthcare won't make one go bankrupt, good service and there's that whole culture of fitting in "the nail that sticks out must be hammered in". With the lack of visual class divide, younger people just don't see the point of working their asses off. They do it at first because that's what they are taught but once introduced to the very rigid and stressful work environment, there is little hope of advancement. I think the depression and suicide is a result of being content - manifesting in frustration and ultimately being unable to deal with the mundane of every day life (combined with other factors like frustration with work and or lack of partners).

I tend to agree. I have lived here for a short time but you get a sense of this just by being around people.