My S0 SS never rebooted by itself, and I have owned it since launch day till January of 2017. My new S2 SS just rebooted once this past weekend, for the first time in its life. It's running wOS 3.1.3, and I wasn't doing anything specific with it at all, just trying to raise my wrist to check time, it didn't respond at all. I tapped the display, still nothing. Then when I was wondering what's going on, the Apple logo appeared and it was restarting. After a few minutes, it started up fine.