At one point in life, I was so enthused about anime that I could watch any series, any time. Just bombed a test? No need for happy anime, just go straight into watching Saikano! Yet, I find that as I get older, or perhaps I should say, more jaded, I can only watch certain anime if my mood allows for it. It's something unfathomable, kind of like when you hear people say they're not in the mood for sex. When you're young and hormone-crazed, you think that can never be the case! But here I am, with the following anime options available: Kaiba, Kanokon, Allison to Lillia, and Crystal Blaze. I like Kaiba, but I'm just not in the mood to watch it right now. Same with AtL and Crystal Blaze.
I find that this does not happen to some anime, however. I'm nearly always up for watching a new episode of Code Geass, Macross F, or Kure-nai. Perhaps it's a testament to the quality of those shows, or maybe it's because they span the range of genres, so there's literally something for every mood.