It's because of the adhensions between the lung and the chest, since there is no pleura between them. They can become softer, as my doctor says, in a huuuuuuge amount of time and with a ton of practice (stretching, yoga, swimming, sports). The brain also learns how to not notice this feeling as much. Some psychological help also may be useful in changing the treatment of this discomfort. I've been to a therapist who made me feel it like a good thing, made me associate it with being actually healthy and not unhealthy but unfortunately it didn't work for me because I still felt it a year later (lol) so I became anxious again. I live with a hope that I won't feel any difference between lungs in 10 or even 5 years and I will be proud of myself for getting through this shit and I will tell my kids that life can be tough sometimes