I agree that if animals were constantly in fear and pain, that's wouldn't be evolutionarily advantageous. So there surely are such mechanisms for switching off pain and so forth.

However you slice it, though, there's a lot of pain in the animal kingdom. For instance, not all deaths by predator are quick and painless. Hyenas often eat their prey while they're still alive.

The question is whether the experience of life on this planet merits the summary word "joy," or whether that's facile and maudlin.