Contrary to received wisdom, research would appear to support the idea that "bad" moods can be good for us, that not only should we accept feelings of sadness, anger, anxiety, etc. as part of being human, they also serve a real purpose - they enable us to see certain things more clearly e.g. we are better at accurate, close observation when we are in a (mild) bad mood.
Now I'm not suggesting that we use that to justify biting somebody's head off because we have a hangover or got out of bed on the wrong side, but it does help to understand a little more about what makes us tick (or otherwise!). Just because feelings are painful or uncomfortable, it doesn't make them wrong or bad, they have a role, and it seems that we are finally beginning to discover what that might be.
There's still a long way to go, but in the meantime, when you notice that bad mood coming on, as well as trying to find out what's causing it (and it really could just be that it's a grey day), you may also want to think about the benefits.
http://theconversation.com/why-bad-moods-are-good-for-you-the-surprising-benefits-of-sadness-75402