I've found this is a very useful exercise when your requirements are much simpler than [popular library], or you need a specific feature that it can't easily be

I've found this is a very useful exercise when your requirements are much simpler than [popular library], or you need a specific feature that it can't easily be patched to support. Write your own, and even if you give up halfway through and use the popular library after all, you'll have learned a lot about how stuff works, and can make better decisions about how to use the library.