Everyone has good days and bad days. There are those days when you wake up energized and eager and then there are those days when you feel you're deserving of a yet-to-be-invented award because you didn't spend the entire time sprawled on your bed, moaning in a low, mournful register. Not everyone is agreeable all the time. Not everyone is disagreeable. People's moods change.
It's easy to forget this as we write our way through our stories. We're so busy focussing on plot and pace and not repeating words and dialogue tags, that our characters' moods sometimes get short shrift. A character who is happy all the time or conversely, ornery all the time is going to get dull fast. Readers may not always be able to put their finger on it, but those characters are going to hit a false note.
Of course, moderation is the key.
I don't worry about this for every character; my small, one paragraph wonders appear, say their bit, trip, and get off the stage. But I do try and remember it when I'm writing through one of my POV characters' eyes. Not every one of them understands why they're in a bad mood, but I know why and it's there. Bad moods make people short-tempered. Impatient. Not wanting company. Bad moods make people retreat to their 'places of comfort'; whether it's a bar or a bathtub. Good moods make people talkative. When people are in a good mood, they physically touch other people more often. Good moods make them walk quicker. They plan activities. They want to be with their significant other or they want to search a little harder to find one.
Let your main characters change as often as the weather. Let them have good days and bad days. The more human they are, the more beloved they will be.