- Injustice. If this is dominant in their world and there isn't much they can do about it, then this would be very good cause for a character to be permanently grumpy at the world about them.
- Illness. Illness can make anyone grumpy. I confess now I'm not the world's best patient. But in your fictional setting, what sort of illnesses could your characters get? Are they the same as we have on Earth or something unique for your creation?
- Hunger. Not having enough to eat is a very good cause for grumpiness. I know I get ratty when I'm on the very hungry side. I thought this aspect of things was brilliantly portrayed in The Lord of the Rings as Frodo and Sam get nearer to their destination and the food is running out. Notice how the tensions racketed up.
- Frustration. (Of all sorts!). The character that is on a quest certainly doesn't have to be happy about it. The stress of that quest should make itself manifest and this would be the obvious way for it to do so.
- Resentment. Especially at the success of other characters. This resentment does not have to be justified.
Grumpy was, of course, a great character himself in Snow White and the Seven Dwarves but here I'm looking at reasons why your characters (or at least some of them) might have cause to be on the cantankerous side.
I'm Allison Symes and write fairytales with bite, especially novels and short stories.