Prisoner of Man. Pikachu is the only one of Ash's pokemons that has not evolved yet over the course of years the show has been on-air. It is infer that Pikachu isn't allowed the freedom to express his true self and venture out into the world because of Ash's strict restriction he has on him.
Matt: Living at home suck when I'm going to college. My parents treat me like I'm still a kid.
Drew: That sucks man, but just be glad your not a Pikachu!
Matt: Haha, your right. Anything is better than that!