They are in constant pursuit of food which is futile since larger dinosaurs keep stealing it from them. The movie offers only a few glimmers of hope for these poor dying baby dinosaurs who are torn apart by rampant dinosaur racism (another theme that's kind of weirdly thrown in there). Until finally, when they're most down on their luck Little foot see's a hallucination of his mother in the clouds and suddenly the great valley appears. Where they are reunited with their parents and everything is wonderful and happy... cause they died of starvation and went to heaven.
Rooter even gives that speech about "we don't all get there at the same time but we all arrive at the same place."
Of course the other possible theme is that they are Marxist dinosaurs, drawn together by oppression. T-rex represents the global elite of a capitalist society, taking what he wants with no regard for the lesser creatures. Bigger stronger dinosaurs take whatever they want and leave nothing for smaller (poorer) dinosaurs. The small poor dinosaurs ban together overthrow the global elite and create a perfect Utopian society where all dinosaurs are equal and have enough to eat.
