The answer to 'why is my toilet constantly running' is that the most common cause is a problem with the toilet's flapper or fill valve. The flapper is the rubber seal that lifts up when you flush, allowing water to flow into the bowl. If the flapper is worn, cracked, or not sealing properly, it can cause water to continuously run into the bowl. The fill valve is the mechanism that refills the tank after a flush, and if it's malfunctioning, it can also lead to a constantly running toilet.
Other potential causes include a problem with the toilet's tank or bowl, a clogged or leaking pipe, or even a problem with the water supply line. If the issue is with the flapper or fill valve, this is usually a relatively easy DIY repair that you can do yourself. However, if the problem is with the toilet's internal components or the plumbing, it's best to call a professional plumber to diagnose and fix the issue.