The answer to 'why is my water line constantly running' is that there is likely a leak or problem with the plumbing system. A constantly running water line is usually caused by a leaking faucet, toilet, or other plumbing fixture that is allowing water to continuously flow through the pipes. This can result in high water bills and water damage if not addressed promptly.
Some common causes of a constantly running water line include a faulty toilet flapper, a leaky faucet, a broken water supply line, or a problem with the home's main water shut-off valve. In some cases, the issue may be with the water meter or the municipal water supply line outside the home.