As a senior associate, I make a point of telling juniors that I'm a heavy wordsmither, and "it's not them, it's me." They still don't love their work being edited heavily, but at least they know it's not intended to be a direct comment on their work quality.
(To a prior commenter's point, before I delete particular things, I do reach out to ask why they were included if I can't follow the reasoning. Often there's a good point that the junior was trying to make but didn't draft as clearly as s/he might have.)
I also feel like supervising attorneys like/feel the need to get that red pen out and say "this is how it's done." I can't tell you how many times I have copied a brief or motion from a partner, used it in my own motion/brief, handed it to them, and get it back with their own words scribbled out and some suggestions.
How do you know you're using the partner's own words? There are any number of reasons they might not like the wording in the prior brief - maybe a client insisted on including it, maybe they couldn't wordsmith the last draft as heavily as they'd have preferred due to timing constraints, etc. Of course, if you are specifically aware that a partner insisted on including specific wording the last time you work with them, and the next time they criticize that same wording, then that's pretty funny.