In the Help Center's How to Answer page, under the "Always be polite and have fun" section, I suggest that we add: "don't make it personal". The use of I and You can make an answer seem more contentious.
I make all my answers personal. Please don't make me delete all my answers!
Making an answer personal doesn't make the answer contentious. Neither of the following are contentious: 'I think using a generator over a list could be better here', 'Your code doesn't adhere to PEP8, Pythons style guide.' It's also easy to be contentious without being personal: "The submitted code sucks."
Making things personal doesn't mean the answerer is contentious, it doesn't make the answer; rude, unhelpful or unwelcoming. You can be an arse whilst using 'I' and 'you', and you can be an arse without using them too. Not using "I" and "you" can help people not come across rude, but to outlaw making an answer personal doesn't solve this.