Honestly, its probably coming from a scarcity mindset, I suspect, with respect. If he is a guy, whose longed/pined for a woman and the woman rejected him or alternatively, had a woman he really loved who “lost the feels” for him and ultimately left him, he might have been the one “crying” to speak and from his perspective, perhaps it is better to be the one moving on to a new relationship and being happy with a new girl…than being the one left behind while the old-gf goes and has a “hot girl summer” or “posts amazing photos of trips” paid for by some new hot guy she’s moved onto – living "her best life '"while he struggles to find someone one. If that’s his lived experience/frame, I could see why he’d want to live what the hot-girl who left him seems to be living compared to his potential loneliness or feeling of rejection.
However, there’s a lot of grey-area and places inbetween. You could have a mutual breakup and both find someone new in a short time or she breaks up with you and you find a new, better woman quickly while she struggles to find a better man than you… these, if he’s a below average or average man though, might not be in his lived experience or something he imagines as possible. But he can imagine the crying-over-me and not me-crying-over-you scenario as he just has to put himself in the shoes of his ex or imagine the scenario reversed.