For animals that are R-selected or, in other words, having many offspring in the hopes that some will survive, the vast majority of the offspring die very quickly. Most species of Fish, Amphibians and many less complex animals do this. 99.9% of them dieing in before reaching adulthood might be a good approximation for some species. A painful death doesn't seem worth a brief life as a wild animal.

It's true that most people wouldn't be functioning optimally if they were not somewhat happy and extrapolating this to other animals who seem to be similar to us in basic emotion, I would agree that an adult wild animal seem like they would live an alright life.