I agree it's probably wise to try and stay cheerful, and although there are limits to how well that can be achieved, I think it's possible to have some conscious impact on one's spirits. I try not to get morbid, with some success I think. I used to do this thing where I'd allow a lot of negative thoughts, and I couldn't feel them whittling away at my mood. I can't honestly say I ever found out for sure that they were doing that, but it seems to be the received wisdom that it's bad for the soul, so these days I keep more of a brake on it.
It seems to help me to establish a positive general manner when I'm with certain people who seem to enjoy my bright patter. Once I'm in the habit of it with a particular person or group, I tend to find it becomes a habit for me to be like that when I'm with them, and so it feels like they cheer me up when they're around, though it can also get stressful if I have to be with them for too long, I guess because I have a dark side too. I suppose it's best not to put on too much of a clown's persona, but just to try to be reasonably bright for most of the time, and to allow the occasional bit of pathos when it's appropriate, otherwise it can start to get rather fake and precarious.