The only things which are intrinsically "good" or "bad" are pleasure (good) and pain (bad). Everything else, including desire, only lead to greater or lesser pleasure or pain
And Epicurus saying in the letter that sometimes we treat the good as bad and the bad as good is a very clear statement and stark reminder of how "relative" those terms (good and bad) really are.
Nevertheless the world throws around those terms (good and bad) as if they were handed down on tablets from Mt. Sinai!