The answer to 'bathtub constantly running' is that the most common cause is a faulty or worn-out drain stopper or overflow drain. This allows water to continuously leak out, causing the faucet to run constantly to try to keep the tub full.
Other potential causes include a problem with the plumbing pipes or fittings, a clogged drain, or issues with the bathtub faucet itself. If the issue is with the drain stopper or overflow drain, this is usually a relatively simple DIY repair. However, if the problem is with the plumbing or faucet, it's best to call in a professional plumber to diagnose and fix the issue.