The answer to 'why is my sink constantly running' is that the most common cause is a faulty or worn-out sink faucet. Over time, the internal components like washers, O-rings, and seals can deteriorate, leading to a constant drip or trickle of water. This is often caused by normal wear and tear, but can also be due to improper installation or damage.
Other potential causes include a problem with the sink drain, such as a clogged or damaged drain pipe, or issues with the water supply lines. In some cases, the problem may be with the sink itself, such as a crack or hole in the basin.