I know someone who says that he is suffering through a 'mid-life' crisis? We were trying to ascertain the source of his feelings for such and could only arrive at the solution that, one day he simply 'wake up' with the realisation that all his dreams of achievement, exotic and exciting travelling, high-earning and conquering had not been fulfilled.
He is just simply, a normal, standard guy, with a normal standard life (which, by the way, we didn't criticise), the odd family holiday etc. But most importantly - that's it!
And it's that adjustment to life which we figured, men would call 'the mid-life' crisis. Women just deal with it, when they have children.