I think it's the difference the software has made to the people who use it. I often speak to people at conferences or they send us an email and tell us that Perch has transformed their business. Perhaps they have been able to offer a CMS for the first time, or it has enabled a more efficient way of working. Often people tell us that they don't spend half their day handholding clients through using their CMS since they started using Perch, that gives them time back for more fun and profitable work.
As a software developer and business owner seeing the thing we make help other people build great solutions and run a better business is an amazing thing. We're not changing the world, but we're helping some folk do good stuff, and I like that :)