How long do you think he has been miserable (I know they will say thoughout the whole marriage), but when did you first notice a lot of dissatisfaction? I know with my husband he'd complain about silly things and be critical, or have very high expections about a lot of things in his life... i.e. job, kids not doing this or doing that, me not do this or that, etc...
There is no arriving, ever. It is all a continual becoming.
