The answer to 'why is my drain' is that there are several common causes for a clogged or slow-draining drain, including hair, soap scum, food particles, and other debris buildup. Symptoms may include slow draining, standing water, or even foul odors coming from the drain.
Some simple DIY solutions to try first include using a plunger to dislodge the clog, using a drain snake or auger to remove a deeper blockage, or using a chemical drain cleaner. However, if the clog is severe or recurring, it's best to call a professional plumber who can properly diagnose and fix the issue.