This really upsets you, doesn't it?
People are talking to a person about them, their life's work, their ideas, their sense of competence, their choices, and one is telling them not to take it personally?
This phrase that people use as a disclaimer to make an excuse to say or do something rude or mean to you to prevent you from having a poor self-image or so there won't be repercussions and make you feel they still like you even though they probably don't.
If you don't think the act of working or living with others is in any way 'personal', perhaps you might be better thinking of a new career: as a taxman or a "cop who gives a fines"? They really don't take things personally.
