Regarding how advice is not taken well by some players… yes, some players are simply that touchy that even offering advice with an ice-cream cone, a box of candy and a poem is going to be taken the wrong way. However, what some people try to pass off as “being helpful” beggars belief. I've seen people say things like “Dude, how did you get this far in the game without knowing how to play your job? You're supposed to be doing “x, y, z”. Or, “you're playing really badly, you need to watch some videos and learn to play right”, “Here's some advice on how not to suck” and other such abrasive remarks (all of which, while paraphrased, I've actually seen in party chat). Then these people are surprised when their “constructive criticism” is met with defensiveness or hostility.
Meanwhile, I've seen people who say things like, “Hey, -insert name here-, would you mind if I offered some tips on -insert role here-?”, or “I can suggest a good rotation that will help your DPS a lot, if you want”, or “Do you have “such and such skill” from “such and such class”? It's a really helpful skill you should try to get. It helps your “such and such” a lot”. I've also seen those kinds of remarks. Not surprisingly, the reaction to those people is 100% more positive. I hope I don't have to explain why.
The point is - before you complain about "people not accepting my advice" or "getting defensive" - consider the way that advice is being offered or stated.
Regarding people who claim that if they're doing (as a random example) 1500 dps, and another is "only" doing 1000 that you were "carrying that person", answer me this: Could you have cleared that content, in the same amount of time, and with the same efficacy without the 1000 dps person being there? If no, then they were contributing, you were not carrying them, and you should promptly get over yourself.
And lastly, if someone's in your group and says “Hey, I'm new, can anyone offer any helpful advice or tips for me?”, if all you're going to say is “Don't die”, or “don't step in the bad stuff, and don't die”… just shut up. You're not being helpful, nor are you trying to be. You're being an obnoxious douchebag. And if you are that person, and the new person makes a mistake, do not bitch them out or try to vote-kick them. They asked for tips/advice… you chose to be an ass.