[KyleGoetz]

たい is used to talk about your own desire to do something
たがる is used to talk about another person's desire to do something
てほしい is used to talk a person's desire for another person to do something

ピザを食べたい = i want to eat pizza
ピザを食べたがっている = he wants to eat pizza
ピザを食べてほしい = i want you to eat pizza