The answer to 'why is my faucet constantly running' is that the most common cause is a worn-out washer or seal inside the faucet. This allows water to continuously leak through, even when the faucet is turned off. Other possible causes include a faulty valve seat and spring, or a problem with the faucet's internal mechanism.
A constantly running faucet can waste a significant amount of water and increase your utility bills. It's generally a simple fix that a homeowner can do themselves with basic plumbing tools and a faucet repair kit. However, if you're not comfortable working on the faucet or can't identify the exact issue, it's best to call in a professional plumber to diagnose and repair the problem.