The answer to 'why is my shut-off valve constantly running' is that the valve is likely faulty or stuck in the open position. This can be caused by a buildup of debris, mineral deposits, or wear and tear over time. The constant running of the shut-off valve can lead to higher water bills and potential water damage if left unchecked.
To troubleshoot the issue, you can try manually closing and opening the valve to see if it's stuck. If that doesn't work, you may need to replace the valve. This is generally a DIY-friendly task, but if you're not comfortable working with plumbing, it's best to call a professional plumber to ensure the job is done correctly and safely.