As a maintainer of a project, you will be one of the few people in the world that knows a lot, maybe most, of the internal workings of your code.
Having that intimate knowledge of internals and general architecture makes you a suitable person for debugging hairy and difficult problems. The problems that are not easy to digest or understand after the first few rounds of questions have been answered.
In an Open Source project that has a lot of different persons involved, it only makes sense that the individual that can do a particular job the fastest and most efficient way, actually performs it. This makes debugging, and then I mean difficult and time-consuming source code researching, a task that ends up on your plate more often than on the less experienced ones.
If you learn to enjoy the challenge of debugging, you set yourself up to appreciate Open Source maintaining more.
The older a project gets, the less time you get to spend on developing new stuff and the more time you spend debugging existing code. Maybe older projects also get more complicated problems that take even more effort and time to debug. Due to legacy and things that were established years ago and maybe not so convenient and clever anymore.