Most of us enjoy the company of others now and then, even though we may also be happy on our own at times. Spending time alone is fine, just so long as it doesn't turn into the only thing you do. The trouble is, although it's generally true that being around people will give you a boost, if you're feeling ropey, the obstinate part of your brain may try to keep you from social situations.
Crazy isn't it? Logic tells you to accept invitations, to arrange to see a friend, or to pick up the phone, but the annoyingly pervasive emotional side of your thinking tells you not to. What to do therefore? Well, perhaps when the opportunity arises to spend time with others on one of these not-so-sunny days, ask yourself 'What's the worst that might happen if I ignored my emotions'?
I guess the answer could be that you might get wherever you're going, then would need to make your excuses and go home. But (a) that's not actually terribly likely, and (b) it really wouldn't be that awful a thing to do if it was truly necessary. So why not try this next time you feel a bit rough? Nine times out of ten it's worked for me.