Important point here: kind and nice are not synonymous. It's easy to conflate kindness with niceness, and the OP (from what I can infer) is advocating the latter, not the former. Kind is not the same as nice.
There are several ways to do necessary, un-nice things in a very kind manner. It is a skill, and for some people requires coaching on how to express themselves professionally and kindly most of the time.
(Qualifying this, because there are occasions that warrant a good moderate yell or two. E.g., the 3rd (rejected) and random PR submitted by a Jr. developer that changes all of the spaces to tabs in the repository, or all of the modules/functions to classes/methods.)
By the way, a lot of the "yells" can be replaced by pre-commit hooks, auto-configure scripts and other things putting pressure on the system instead of a person.
There are several ways to do necessary, un-nice things in a very kind manner. It is a skill, and for some people requires coaching on how to express themselves professionally and kindly most of the time.
(Qualifying this, because there are occasions that warrant a good moderate yell or two. E.g., the 3rd (rejected) and random PR submitted by a Jr. developer that changes all of the spaces to tabs in the repository, or all of the modules/functions to classes/methods.)