Melancholy is good. Even though they won't quite admit it, most people enjoy melancholia more that the happy stuff. It's almost like a drug.
You take some and it gives you a bitter-sweet pain. But it's 'pain' and not 'happy', so you feel bad and sorry for yourself. It's so tempting, even addictive, to stay in that mood. You mildly feel like a victim, so the pressure to shake it off and dive into work, or take care of people, etc etc weakens a little bit. You tell yourself that you can slack off and do nothing. Even when you do manage to break out of this cycle of self-pity and inertia, you secretly crave to get back to it whenever you can.