[flom]

The problem with the analogy between music and math is the free-will aspect. Everyone is FORCED to take math, whereas most people are NOT forced to play an instrument (at least in the US). Everyone knew a couple of math wizzes who usually were not the one's asking "why do I need to learn this?" These kids would probably do math even if they weren't forced to, and indeed, many of them go on to pursue a higher education in mathematics, completely voluntarily. On the other hand, many kids whose parents force them to play an instrument often complain and ask "why do I need to play an instrument?"

tl;dr: If you force someone to do something, and you don't want to make them resent the thing you're forcing them to do, you should explain to them why you're forcing them to do it, i.e., why it's in their best interest, whether it's math, music, learning a language, etc.