Someone online asked this question; and to be honest, I couldn't be bothered writing a "whole big thing" in reply to it. In any case, here's some of the possibilities that I came up with...
Maybe she keeps asking him "why", and so he answers.
Maybe he thinks it's better to be honest and just say what made him unhappy. A lot of people are raised to believe it's "superior" to be honest, rather than "candy-coat" or lie.
Maybe he gives her credit for "being a grown-up" and underestimates his own potential of having any impact on her.
Maybe (and this one is a big one for a lot of guys, I think) he thinks if he offer "constructive criticism" she'll appreciate it, and maybe even "fix herself" so the next guy won't leave her too.
Maybe he's clueless when it comes to emotional matters and relationships (a lot of guys are this).
Maybe he's under the impression that she's plenty confident (which may not be the case), and it doesn't occur to him that she may be hurt by his words.
Maybe he thinks she's over-confident and needs "a reality check.
Maybe she has been emotionally abusive (either intentionally or unintentionally), and he needs to air his own grievances (which he may have been keeping to himself for a long time).
Maybe she has been more needy than he could deal with, and he needs to express what he's kept to himself for a long time.
Maybe she's otherwise (from above) "put him through the ringer" in some way, and he needs let her know why he won't take it any longer.
Maybe he never really thought much of her for any number of reasons, real or imagined; he came to be angry at her for not being what he wished she were (or what he thought she should have been), and so he wanted to vent some of that anger and/or let her know how little he thinks of her - for no reason other than that he is someone who either settled, or else expected the unreasonable (like perfection) - and trying to hurt is just part of his way (of being someone who isn't fit for a relationship).
Or maybe, he's not a "high-end-of-the-spectrum-narcissist" but he's more of a narcissist than is healthy; and because of that he had seen her as "less than quite an equal human to him"; and as part of that, he had been expecting her not to show signs of being a separate, thinking, feeling, individual. When this happens self-centered, narcissistic, people get angry and respond by trying to be hurtful.