I think we're "supposed" to destroy ourselves for the sake of advancement for a few reasons:
To preserve the myth of Western egalitarianism. Supposedly, we have a classless society. Anyone can make it if they just put in the effort. Mind you, this isn't true: plenty try and fail, and even those who succeed sacrifice their life to advance from one class to another. But we're supposed to believe that the only reason we don't have certain things is because we don't want it bad enough, and/or lack the discipline to succeed. The goal: get people to always look inward for the source of their suffering, and fail to recognize the very real economic parasitism that prospers at our expense.
A manifestation of that old but persistent notion that to be righteous is to suffer. If you are happy, if you aren't suffering, you must be doing something wrong. Good food tastes bad. Good exercise hurts. Good work is miserable. To be good in spirit is to mortify the flesh. Put on your hair shirt, run five miles, drop and give me twenty, and then complete a twelve hour shift. Sleep is for the weak.
What offends people who take this advice more than anything is someone who hasn't lived this way, and yet is happy when they are not.
I think the main reason people say it is because growth happens during times of discomfort. If you're only ever comfortable, you'll become weak and die early because your body will fail faster.
My sister was diagnosed with ASD prior to transitioning, and she went the complete opposite way from the "keep yourself comfortable" mindset.
It can absolutely be fun to be comfortable for a while, but eventually the other shoe will drop and it's going to lead to misery and deteriorate your mental state.
I have a roommate with ASD that I've really been trying to drive this home to, and completely honestly, I feel incredibly guilty like I'm enabling him by allowing him to live with me for cheap without imposing any expectation that he improve himself.
Discomfort stimulates growth, but the actual growth happens during periods of recovery. That is true of the body, and I have little doubt it is true of the mind, as well. I'm not saying people should never step out of their comfort zone. But just like we shouldn't be judging people at the gym because, from our perspective, they should be able to do more, we should be extending compassion to those of us who have difficulties in the mind, particularly considering we can only know our own perspective, not theirs. I mean, you wouldn't expect a guy in a wheel chair to be doing leg presses, would you?
Yeah I agree. Getting out of your comfort zone simply conditions yourself to feeling uncomfortable, but simply doing it isn't going to get you the things that you want. If you tag along as a fifth wheel to parties and stand around but don't actually like being there, you're out of your comfort zone but you're not actually gaining anything. You aren't going to make friends or find a girlfriend by standing in the corner.
You also need to approach with compassion and I agree that we should moderate our expectation. Someone who has been a shut-in would be making a lot of progress just by going out to a party once.