Why does criticism from a partner hurt more than from anyone else?
Because of who they are to you, what you have told them, and the fact that you cannot walk away from the relationship afterwards — three things that combine to make the same words land far harder.
They have information nobody else has. A partner knows your insecurities because you told them. Criticism from someone who knows exactly where the soft spot is lands on it, whether or not they aimed. This is the cost of the intimacy, not a failure of it.
Their opinion is weighted differently. We calibrate self-perception against people whose judgement we trust most. A stranger's dismissal can be discounted; a partner's cannot, because you have already decided they know you well and are worth believing.
It threatens the relationship, not just the ego. Criticism from a colleague concerns your competence. From a partner, it can feel like evidence about whether you are loved — an entirely different and much larger question. The brain treats threats to close attachments seriously, and rejection by someone you depend on activates systems overlapping with those involved in physical pain.
You cannot leave the room and forget it. Criticism from an acquaintance ends when the conversation does. From a partner, it is waiting at dinner.
Accumulated history. The same comment carries the weight of every previous version of it. "You never listen" said once is a complaint; said for the fifth time it is a verdict, and it is heard as one.
What the research adds. John Gottman's work identifies criticism — attacking character rather than describing behaviour — as one of the strongest predictors of relationship breakdown, precisely because it is heard as a statement about who you are. The distinction between "you left the plates out" and "you're so inconsiderate" is the difference between a complaint and an indictment.
Negativity bias compounds it: negative interactions weigh more heavily than positive ones, which is why ratios of positive to negative interaction predict outcomes better than counts of either.
What actually helps: describing the specific behaviour and its effect on you rather than characterising the person, raising things early while they are small, and — for the person receiving it — asking what they want rather than defending immediately.