Although I have never had sloppy 2nds from my wife I enjoyed them many times in my premarital days when I was in the navy. I love the feel of sliding my cock into a pussy or ass that is already filled with cum. I have been trying to talk my waife into fucking other guys while I watch. I would then fuck her cum filled cunt not caring if it was 2nds or 10ths.
I think it is a powerful desire because you are allowing another man first have your woman, you see her enjoying herself with this other man. Maybe because he has a bigger cock, or is younger or whatever, and your girl may not care about him, but is enjoying the sex.
Now I have presonally only had (semi-)sloppy seconds a few times as far as I knew. My ex when she was sleeping with another guy would have some good day time sex, his very large cock would stretch her out and then when I got there a few hours after the fact I would easily slide into her used wet, very aroused pussy. She seemed so much more sexual those times and I think that has a lot to do with why seconds is so amazing for the committed partner.