As I read this post by Quân I recognized myself.
…although I can often be of joyful nature, I think d. is right. Without a specific reason to be happy, I usually revert to a state of melancholy. I wasn’t always like that and I’m not certain what triggered the change. Sometimes I feel guilty about it. Some people manage to always have a smile across their face and I find it remarkable. I can’t do it.
Although where he says he didn’t use to be like that I think I’m actually getting better.