I think that the heart of the issue is that more men nowadays feel that they just don't matter. Not to society nor to anyone else. This leads to a situation where they will simply sacrifice less and try to enjoy more.
So why would men feel that way?
My initial thoughts were that perhaps it is due to the recent demonization of men and the removal of some of their privileges, while the burdens (selective service, unfair family court) remained. While I think that these could be contributing (accelerating) factors, I don't think this is at the heart of the matter. We see similar trends in other countries, where lots of (young) men are essentially checking out from society. Japan is an example of a country where this problem has likely progressed further than in the US. My initial thoughts don't fit, because they couldn't explain why this same process would happen in countries with a pretty different culture from the US.
I'm stumped. Perhaps it's somehow linked to the falling testosterone levels in men that we have observed?