Building a new product is always fun. You draft ideas, generate wireframes, mockups, prototypes, you build your app, you tweak it, you release it. In the case of software and web applications you also get to update it and make it better. If it’s hardware, you work on a 2nd revision to be sold a year later to people who didn’t adopt early (jab at early adopters).
One of the most interesting things is how users actually use the product you make, if they use it at all. Do they use it a little or a lot? Do they use it as intended? Do they find things missing? To robust for their taste? Or do they just find uses and modifications that all the engineers involved never in a million years would have contemplated?