Alright, I'll work with your premise and re-phrase "Don't get lasik, I like your glasses" "Don't get hair treatment, I like your widows peak." or "Don't lose more weight I like your dad bod"
I think penis size is often alluded to in order to imply attractiveness or used as a measure of attractiveness. Underwear models have a variety of methods of making their bulge look more impressive, and the vast majority of paid performers in porn are well endowed. Advertising is very targeted these days, if you are not its intended audience you may not notice it but my point wasn't about dicks, it was about perception. What you think of as a commodity with an industry around it is still part of an individuals body to feel insecure or comfortable about, and also make changes to as they see fit.
I spent seven years smiling with my mouth closed while everyone around me told me a space in my teeth wasn't a big deal, that it gave me character, that it wasn't noticeable. I felt like I shouldn't want to change it and yet I was still so insecure until I finally went ahead and did something about it when I went to university. I wish I had done it sooner, it changed my life to smile without covering my mouth with my hand, or immediately feeling embarrassed.
If someone else wants a boob job, their nose done, botox, or hair extensions because they really don't feel confident the way they are, telling them they shouldn't can make the decision less comfortable.