The answer to 'why is my shingles water stains' is that water stains on shingles are typically caused by a leak in the roof system. This could be due to damaged, missing, or improperly installed shingles, as well as issues with the roof's underlayment or flashing. The water stains may appear as discoloration, darkening, or even mold growth on the shingles.
If you notice water stains on your shingles, it's important to address the issue promptly to prevent further damage to your roof and the interior of your home. Depending on the extent of the problem, you may be able to fix it yourself, but in many cases, it's best to call a professional roofing contractor to inspect the roof and identify the root cause of the leak.