Solve Your Own Problem
Build something you personally need and understand deeply.
Explanation
The best startups often come from founders solving their own problems because they understand the pain viscerally and can spot when they've built the right solution. You're your own first customer, which accelerates learning and iteration. You'll also have natural empathy for other users with the same problem.
Real-World Example
WhatsApp founders were frustrated with SMS costs when traveling. Airbnb founders needed money and had extra space. GitHub founders wanted better code collaboration tools. Stripe founders were developers tired of payment integration pain. Drew Houston built Dropbox because he kept forgetting his USB drive.
How to Apply
Inventory your daily frustrations—which ones do others share? Start with problems you experience weekly, not annually. Validate that others have the same problem before building. Stay close to the problem even as you grow—don't delegate customer understanding. If you stop using your own product, investigate why immediately.