Mostly, it's my brain's fault. The stupid thing keeps me up until the middle of the night, bombarding me with random nonsense. Thoughts rattle around in my head like ballbearings in a blender, and it's hard to shut it off long enough to fall asleep.
Sometimes, one of the ideas will be strangely compelling. It might be a glimpse of something I've seen or heard: a grainy mind's-eye image of a place or a person. Maybe it's a few seconds of a conversation between two people, where someone says something interesting. A few frames of a movie that doesn't exist, showing some movement or activity or location that catches my interest.
The memorable bits and pieces stick around. If I spend enough time thinking about these things, I might start to form connections between the disjointed thoughts. Maybe that one interesting line of dialogue was spoken by that one interesting-looking character. Perhaps the grainy picture of a location can be the place someone comes from.
Over time, I might assemble enough bits and pieces that something coherent starts to emerge. A new character, for instance, that I first met as a face or an expression or a single line of dialogue. They start to form into a person and, if I'm really lucky, there might be an idea for a story they can take part in. I've got a few characters in my head that I'm really happy about, but I don't have anything for them to do; I don't have a story to put them in.
So if you ever see me slumped in my chair with my eyes closed, rest assured I'm working really really hard, trying to grab at the ideas whizzing by in my head. It's another vital part of the process.