applied mathematics can get very messy: it requires performing a bunch of computations, optimizing the crap out of things, and solving tons of equations. you have to deal with actual numbers (the horror), and you have to worry about rounding errors and stuff like that.

whereas in theoretical math, it's just playing. you don't need to find "exact solutions", you just need to show that one exists. or you can show a solution doesn't exist. sometimes you can even prove that it's impossible to know if a solution exists, and that's fine too. theoretical math is focused more on stuff like "what if we could formalize the concept of infinity plus one?", or "how can we sidestep Russel's paradox?", or "can we turn a sphere inside out?", or The Hairy Ball Theorem, or The Ham Sandwich Theorem, or The Snake Lemma.

if you want to read more about what pure math is like, i strongly recommend reading A Mathematician's Lament by Paul Lockhart. it is extremely readable (no math background required), and i thought it was pretty entertaining too.