Thanks for this thread, Maggie!
Who can explain the weird compulsion to do this stuff?
I don't always feel motivated, but if not I try to make time to be at my desk and once there, it somehow starts.
As others have said, the burning desire to tell stories, and communicate what your characters are up to, are prime motivators. When I first (re)started on this journey a few years ago, I wondered if the stories would come, but now I'm swamped! The key to unlocking your story vault is to start on a couple of things and, while you're working on those, note down every idea you have.
Ultimately, it's all about print for me. If a story doesn't end up in print then I consider I've wasted my time (even though I probably learnt something on the way). What's the point of a story without an audience? So I get pretty pissy with unprinted material (in case anyone hadn't noticed

).
The time commitment is huge and that has to be justified. If I'm going to take time away from family, work (nearly all my leave) and other commitments, then it had better be productive and it had better be worthwhile. For that reason, as Bluetoaster has said, I've had to learn to draw like a machine - no excuses, no stuffing around - on a regular basis and often (particularly with deadlines involved).
What frustrates me when I am feeling creative is so much time away, where I have to pursue other necessary things (such as a dayjob). I gave up music (playing and writing), maintaining the garden and nearly all TV watching to make the time to write and draw comics.
I could imagine giving it up, but only if I immediately took up another creative activity again, such as writing and recording music.
But I do it because I have to.
Cheers,
Ian T.