Mid-Life Crisis - and/or the Seven year itch.
This will come on whichever is working fine - say if you have a great career - you will start thinking negative on this .....And if you have a great relationship going fine ... ...doubts start creeping ....
On the other hand, say you have less money, your focus will be on how to increase your revenue and so the Mid-Life crisis or any other thing will not affect you.
So the secret is, always better to stretch your goals and keep on working on it . Don't be idle -as they say - Idle mind is the place of Devil ..