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Why should this LW be considered creepy? It wasn't as if she was his high school girlfriend. They were married for what sounds like around ten years. They have a child together. You don't "just move on" from a ten-year-marriage, especially one involving a child, from which you never can move on. Anyone who can, without at least occasional regrets, either is not human, is abnormally well-adjusted, or both. (Or escaped the marriage from hell, in which case you have your own special set of traumas.) And you're probably always mapping your first marriage onto the second and wondering how not to make the same mistakes--wondering, "was it me? Or was it her/him? How do I not screw up this one?"
Sometimes we focus on little things to avoid having to confront bigger issues. What he's really asking is, "Why does she seem to want to please this new guy in ways she didn't want to please me when we were married? Is there something wrong with me?" Because the fact is that she has grown her hair longer. Yeah, there are all kinds of possible reasons (and thanks, Nat, for the long-hair tips!) that could have nothing to do with him. But then again, it could. Not because he's inherently a loser, but because they just weren't good together.
I don't see anything wrong with having a friendly chat sometime over coffee, where he asks her, a little sheepishly, this question that's been nagging at him all this time, and makes clear that he wants an honest answer...and is prepared for whatever comes out. I suspect that whatever it is, it will translate, "Because we just weren't very good together."
And then he'll see, hopefully, that he and his second wife *are* very good together, and realize that a failed marriage does not mean two failed people.