Grandpa Longneck is a character everybody looks up to. His status in Great Valley society is closer to that of a leader than the position of any other dinosaur. He is challenged by Topps sometimes, but then it becomes clear Topps is not up to the task. His wisdom and calmness makes him look like an unmoving tree in the storms of life, a tree everybody else can grab on. Grandpa crying would probably ruin that image somewhat, but to the movie maker's credit Grandpa is not actually shown as an incarnation of perfection, so the fact that he doesn't cry still doesn't make him too different from the other characters, at least in my opinion.