The answer to 'what causes faucet' is that there are several common reasons why faucets may start to malfunction or leak. The most frequent causes are worn-out washers, buildup of mineral deposits, or loose or damaged internal components like the cartridge, valve, or O-rings.
Over time, the rubber washers and seals inside the faucet can become hardened, cracked, or worn down, which allows water to drip out. Mineral deposits from hard water can also build up on the internal parts, causing them to stick or not function properly. Additionally, the faucet cartridge, valve, or O-rings may become damaged or loose, leading to leaks around the handle or base of the faucet.