Why did they make azurill look a bit worried in gen 4.2 and then in gen 5 they made azurill look like it's about to cry, before it was so happy.
The real answer is because you need a high friendship with them to evolve so thats why the evolved form (Marill) looks very happy.
Gamefreak just wanted it that way. It is not expalined in pokedex entries, but as it evolves it expresion goes from sad-happy-joyful. Besides you can't see it from behind and it just makes your opponetnts go easy on you XD