The answer to 'why is my water line constantly running' is that there is likely a leak or problem with the plumbing system. A continuously running water line is a sign of a larger issue, such as a broken or leaking pipe, a malfunctioning water valve, or a problem with the water heater or other plumbing fixture. This can lead to high water bills and water damage if not addressed promptly.
Some common causes include a leaking faucet or showerhead, a running toilet, a broken water line underground, or a problem with the home's main water shut-off valve. It's important to inspect the entire plumbing system to identify the source of the issue and make the necessary repairs.