I learn something new every day. Even if I have to read a sloppy doc for some library I have to use, I learn about what annoys me and think about how I could do it better.
In my opinion, that's one of the hard parts of the job: we constantly have to learn. But at the same time, it is what makes software development awesome: we keep our brain occupied, which is good for our health and passively improves our ability all the time.
That's also why I tend to see room for improvement in a lot of different situations all the time, even though they might not even be software-related :D