Nothing ages you quite like Quantum Field Theory. One day you're a bright-eyed undergrad thinking physics is just F=ma, and the next you're staring into the mathematical abyss where particles are just excitations in fields and everything you thought was solid is actually a probability cloud.
The transformation from "I understand the universe" to "I understand that I understand nothing" happens at approximately the speed of light squared. And that's not even accounting for the sleepless nights trying to calculate Feynman diagrams while your non-physics friends are out living their lives without existential math crises.