I provide advice if there is a wipe or if I can see something blatantly wrong, like someone wearing totally the wrong gear for their class.
That said, I think the biggest problem is that people who are not mentors forget that mentors are people too. Yeah, when you sign up you agree to be a example of good behavior to your fellow players, but that doesn't mean you never get frustrated or tired. Mentors make mistakes. Mentors can get irritated when others play ridiculously poorly (and ignore friendly advice to fix it). Mentors can be new to content or forget old content. Sometimes they may not notice another player is even having a problem that could benefit from advice. Sometimes just asking if someone wants advice is enough to set people off.
Expecting mentors to do exactly what you want them to do exactly when you want them to do it is what causes these issues. Realistic expectations, where?
That said, I don't really encounter this particular issue on the daily. I've met plenty of people who ignore desperately needed advice, but very few who actively lash out at you for offering it.
