One of the worst things about depression is all the sneaky ways in which it reinforces itself. You can’t function at full capacity, which is a direct problem, but also makes you feel lazy. You sometimes don’t have the energy to deal with people, which isolates you and makes you feel like you’re offending them. You just plain feel awful and feel guilty about inflicting it on the people around you. It seems like everything hits you twice.
Therapy can help with both aspects, but I think it is particularly useful in helping you recognize and deal with the secondary parts of the problem.