Why math?
Math is akin to learning as running is to sports. It's essential to performance across all different domains. People avoid math for the same reason people avoid running - it's hard work. But, it should be essential to all learning. Math teaches you how to solve problems. How to stay with a problem, determine what it is asking of you. To concoct a strategy. To implement said strategy. To see your strategy fail, figure out exactly where it went wrong, and fix it. It's the most basal form of problem solving, and learning how to solve problems will always be useful. I also like solving math problems. Getting lost in a difficult and conceptually fascinating math problem is akin to to being engulfed by the world of a good book or movie. You are fascinated by the characters, how they interact with each other, and the world that is built around them. Everyday worries are forgotten as your mind is completely occupied.
Why weightlifting?
As a kid, I was also very skinny. Compared to my brother, who was swim and water polo captain, I was no where near as strong or athletic as him. I spent lots of years insecure and unhappy about this fact, until one day my senior year of high school I decided to change. It was my first real taste of the addicting feeling of progression. Everyday a small improvement, kaizen. The fascinating thing is that each week I got a little better, not always linearly, but consistently.Since I started lifting about 3 years ago, I have made substantial progress, and turned one of my biggest weaknesses into a strength (see here). I have also applied that inertia to new domains, academics, personal relationships, and math. Weightlifting helped me realize that I am capable of great change. If I dislike something about myself, I can simply choose to no longer do/be that thing.