It's common in every mmo that has healing.

When you pug, sometimes you get grouped with people who are not very bright, and most people who aren't bright don't realise it.

So they don't do a good job of playing their role, and rather than realise this all they get as far as in terms of critical analysis is: I'm dead, my health bar is empty so that's why, and it's the healers job to keep my health bar full. So it's the healers fault.

The idea that its a shared responsibility to stay alive is too complex for their brains effort/education/level of exercise.

Or alternatively they are smart enough to understand it, but also way too egotistical and immature to accept to their share of the responsibility because they can't handle the ego blow of contributing to their own lack of success.

Sometimes its both.

Sometimes the problem is actually that the healer was bad though, but not as commonly as the above things.

To cover yourself from that eventuality make sure you know your craft, and know how well you are doing objectively by comparison to others in your craft and by listening to constructive feedback.

It's possible that a healer can be carried in non top level content, so make sure that isn't you also (not that I'm saying it is, but I want to be even handed here.)