The point of Mystery Incorporated Freddy, honestly, is that he's not a different character from "original" Freddy. And that, when contrasted against a well-rounded cast of individuals with desires outside of mystery hunting, Freddy as a person doesn't make much sense--the same way Scooby-Doo, as a franchise, increasingly appears deficient the more competent its contemporaries become.And, of course, there's another layer that this is a communal fiction, with its participants having differing opinions and differing amounts of information about the characters they're playing. And there's the other other layer of differing versions of Scooby-Doo itself - original Freddy is a different character from Mystery Incorporated Freddy, who is a different character from live-action Freddy as portrayed by Freddie Prinze Jr.
Like, guys, this was airing on the same network as Adventure Time. Deep and vibrant and filled to the brim with emotional depth. Scooby-Doo is traditionally not capable of any of these things, and so Mystery Inc. internalizes all that into Freddy. (And let's be honest, doing so isn't really displacing any sort of established persona for Freddy the same way it would be for the rest of the cast.) Freddy cannot express emotion because he feels it devalues him somehow. Freddy cannot focus on anything beyond mysteries because it frankly wouldn't occur to him. Fred cannot notice that his friends are deeper people because he is a living, breathing metaphor for the things people expect of Scooby-Doo.
Don't get me wrong--Freddy grows and evolves over the course of the series, because he exists in an environment that allows that and thus is proof that the franchise doesn't have to embody those flaws. But he's starting from a base setting of basically just being Fred from Where Are You?.
