It can be grief for the loss of some of the abilities that you used to have but are gone now because of age, fractured marriage or relationship, or career options.
It can be grief for missed or neglected opportunities and life choices that are now too late to pursue or return to.
It could be a combination of all of these things which can make you feel like you have lost everything.
Even if you haven’t suffered any of these losses, you can still feel depressed. Stress can go on for too long. Even positive changes such as a promotion or a new home, can make you feel depressed from the loss of the familiar.