Math is mind-boggingly huge. We've been at it for thousands of years. The area of our knowledge is enormous. But so is its boundary. We aren't running out of problems any time soon. Including "interesting" problems.

> These days, I can look at any of these journals and find at most one or two papers that are even remotely amusing, and algebra was my specialty. On the other hand, I can take a journal in biology like the Journal of Animal Behavior and still find quite a few papers in each journal that are interesting to me even though I’m not even a research biologist! Keep in mind I still like mathematics a lot, and I still enjoy algebra.

I'm sure that if you had more than passing knowledge in animal behavior, you would also find most of the papers dull. Learning completely new things is of course always a blast. Learning about the latest bleeding edge advances in a field where you already know a lot is not as exciting. I'm not sure what point you were trying to make there. When I read papers in physics I'm always thinking "holy shit electrons are so cool and crazy" because I'm always discovering something new at basically every paragraph. But for an expert the novelty eventually wears off.