The answer to 'why is my drain constantly running' is that there is likely an issue with the drain valve or stopper in your sink, tub, or shower. This could be caused by a faulty or damaged drain valve, a problem with the drain linkage, or debris buildup preventing the drain from closing properly.
Some common symptoms of a constantly running drain include water slowly draining, a gurgling sound, or water continuously trickling out of the drain. In many cases, this can be a simple DIY fix by cleaning the drain or adjusting the drain mechanism. However, if the issue persists or you're unable to identify the root cause, it's best to call a professional plumber to inspect and repair the drain.