The answer to 'why is my shut-off valve constantly running' is that the valve is likely faulty or stuck in the open position, causing water to continuously flow through the pipes. This can be caused by wear and tear, mineral buildup, or a mechanical issue with the valve itself.
Some common symptoms of a constantly running shut-off valve include hearing running water even when no fixtures are in use, seeing water dripping from the valve, or noticing a higher than normal water bill. To fix this issue, you may need to replace the shut-off valve, which is usually a straightforward DIY project. However, if you are not comfortable working with plumbing systems, it's best to call a professional plumber to diagnose and repair the problem.