When you haven't got anything else to do... when you haven't got anyone else to talk with... when you're afraid to be alone and afraid to be together and afraid to let the chaos in your head
suck you down into your own fears...
You do something. You play solitaire, or you get into a fight. Or you do drugs. Whatever it is you do, you do it to keep the depression away.
Some people lift weights, or otherwise work out. Others work in their garden, or geek or code or otherwise concentrate on some task... so they don't have to be alone with their thoughts.
Because if they didn't, they're afraid that they'd go stark raving mad.