Why is my phone turning off and on by itself?

This usually points to a few common problems. It could be a software glitch from a buggy app or a recent update. An old, failing battery is another big culprit. It just can't provide steady power anymore. Your phone might also be overheating, which forces it to shut down. I'd start by uninstalling any new apps. If that fails, it could be a hardware issue like the battery, which would need a professional repair.