Volunteer work is a good thing. Doing your job well is a good thing. Criticism of the second is not an attack on the first.
Imagine this scenario for a minute. You are working as a volunteer in a charity store. A co-worker, also a volunteer, has made coffee for all. Instead of your favourite hot beverage, they serve you a hot cup of freshly brewed rabbit droppings. Do you smile and swallow, or do you spit it out? Are they exempt from your criticism due to being a volunteer? "I am doing what this out of the goodness of my heart", they say. "How dare you criticise me?"
The point of this contrived and rather overstated example to drive a wedge between the good, the altruism
of doing unpaid work, and the good, the craftsmanship
of doing something well. It's a point that is seldom understood. It is an easy mode of thought to slip into that nobody has a right to criticise a volunteer, that said criticism is an unwarranted
attack on your very volunteerism. Maybe it isn't. Learning and growth is better done by listening to feedback.
So next time you feel pained that someone is daring to criticise your altruistic volunteerism, stop and think for a minute. The pain that you are feeling may just be your own knee jerking up to hit you in the face. Don't be a jerk.