The answer to 'why is my shingle replacement' is that there are several common causes for needing to replace asphalt shingles, including age, weather damage, poor installation, and improper maintenance. Symptoms may include curling, cracking, blistering, or missing shingles. The best solution is to have a professional roofing contractor inspect the roof and provide recommendations for repair or full replacement.
Attempting a shingle replacement as a DIY project is generally not recommended unless you have prior roofing experience. Roof work can be dangerous and requires specialized tools and skills to be done properly. For most homeowners, it's best to hire a licensed, insured roofing company to handle shingle replacement and ensure the job is done safely and to code.