It's not just a matter of sadness. It's a matter of sorrow and grief which is something a lot of disney movies like to gloss over. Bambi's mom dies and boom hard cut to spring. Mufasa gets Hamlet'd and Simba runs away and we get a happy song about hedonistic cynicism in a bright colorful montage. They don't really dwell on the characters being absolutely miserable. And in fairness I think this is at least partly for the sake of time. Bambi only had 70 minutes and Lion King was only a little longer which doesn't give you much of a middle act but even with that I think they do tend to just give it the minimum. And maybe Don Bluth goes too far in the other direction. It's been a while since I watched American Tale but I don't remember it being a happy time and All Dogs was a parade of misery and character flaws biting people in the ass. But of the two I think I do lean more towards Bluth, which just felt more real I guess.