I was reading through some of the old posts on this forum, and I came across one that mentioned Littlefoot's depression (or sadness, as it were.) I wonder how much Littlefoot thinks about his dead mother, even though she is only mentioned in four movies (I have not yet seen movie 11, but I don't think that she is mentioned there.)
The potent scene in the ninth movie was probably when Littlefoot mentioned his mother (about why he does not have a brother.) It appears that when he is not doing anything, or thinking about anything, his thoughts return to those of his dead mother, which is why he became so unduly sad when his friends were all occupied in the ninth movie.
Another sign of this is in the tenth movie (during that awful second song.) During that song, we see a shot of Littlefoot looking sad (thinking about his mother, perhaps), and we see his father (whom I don't like) cheering him up.
