do you go back and re-read your own old stories? How often?
I do go back and read my own fics from time to time. It's kind of a random thing, no real pattern.

Is there a particular concatenation of circumstances that causes you to do so?
Sometimes I just get the urge to re-read one of them. Sometimes it's because someone leaves a comment, or maybe something I'm currently reading or writing reminds me of a particular story or scene. Sometimes I do just because I feel like it. I have a lot of fanfic as well as all my own (aside from drabbles) loaded on my calibre program so it's very easy just to pop one open and read when I feel like it.

How does it make you feel when you do? How do you find that you react to them -- do they feel like your own work or someone else's?
In general I do like my own stories. Although of course there are always instances when I think 'oooh I wish I'd done that a bit differently'. For the most part they do feel like mine but once in awhile (especially if it's something I haven't read in forever) I'll think 'This isn't bad, did I really write it?'