why does my apple watch face keep changing?

No problem. If you‘re like me and only use a few faces you can go into the Watch app and remove the ones you never use. This only takes them from the watch, so you can put them back later if you change your mind.

The most likely cause is that you are accidentally swiping across the watch face (this is the gesture to change the face) by sliding your finger instead of tapping one spot. Observe carefully, and see if that fits for when the face changes.