Nothing exists in isolation. Our mentality influences our physiology and our physiology influences our mentality.
The next time you're feeling a bit blue, smile at the next person that you see - especially if that person is you seeing yourself in the mirror.
You'll find that the more you smile, the happier you will feel (something that has actually been scientifically studied). And the happier you will feel, the more you will naturally smile.
It's ok if the smiling feels forced at first. Just keep smiling. And if you do, it won't be long before you completely forget about whatever it was that was holding you down!