I agree that their friendship is like few others, and different from most of the others on the show, and I attribute that partially to the role Mac tries to take on himself with regards to Bloo, that being that he also wants to be a "father figure" to Bloo. What I mean by that is the fact that Mac is the one constantly trying to teach Bloo what's right and wrong when Bloo's doing something wrong (that is, being Bloo). While I'm certain that other kids occasionally do this as well, I've always felt that Mac does it more than most others, though whether this is his own personality or just because of who Bloo is is uncertain.
And that's the thing: while Bloo obviously needs someone to set him straight at times, I'd like to see Mac get to act like a kid more often like he did in the first season and in
Squeeze the Day. I kind of would have liked to see him loosen up in
Bus the Two of Us, but I can kind of understand why that didn't happen.
Honestly, though, they need more episodes of Bloo and Mac hanging out as the main story with a possible B-Plot.