I feel a lot of it lies in expectations. When you are brand new to the game, it makes sense why you would make mistakes, since you haven't had time to get used to how the game works. But by the time you've hit level 80, you are expected to be used to the basics. The occasional mistake will still happen, but you shouldn't die to the same mechanic over and over again. When you die, you are expected to try working out what killed you. Learn to pay attention to what the boss is doing. Not only to help you do better agains that particular boss, but also to have an easier time learning the mechanics of future bosses. If you struggle with figuring out a mechanic, do not be afraid to ask!