When I can't buy it in a reasonable way lol
Simply wanting to save money is a valid enough reason to pirate. The only time you should have any second thoughts is if its a product you REALLY want to see more of or if its made by a smaller group that could really use that money.
Even then though, you can always help without spending money. Easiest way is to spread the word.
You enjoyed that game?
Tell others its a good game worth getting. In many cases, that might help more than buying the game and saying nothing about it.