After experiencing difficulties with their initial product and investor interest, the founders of Lago discovered their successful business idea by reflecting on their past corporate experiences rather than chasing trendy topics. Inspired by Peter Thiel's concept of "secrets," which are overlooked problems that appear solved, they identified billing as a significant issue from their time at fintech company Qonto. This insight led to the creation of Lago, a platform designed to manage complex billing systems, especially for mid-market to enterprise-level companies, without the need to be their own ideal customer profile (ICP). By understanding the problem space deeply, they realized the importance of addressing needs that they had experienced firsthand, even if they weren't currently facing them in their own operations. This approach echoes the success of companies like Auth0, which built solutions for complex problems observed in larger enterprises, demonstrating that the best startup ideas often arise from personal industry insights rather than popular trends.