Why does he keep looking up ex?

I’ve caught my boyfriend looking up his ex multiple times and it’s always the same ex. I asked him and he said it’s because she told him he would never be anything in life says he often checks her page cause he likes seeing that she isn’t going anywhere in life. Is it that serious? or is it just a sorry attempt of a coverup? I mean how can he tell she’s not doing anything with her life just from pictures people don’t post everything on social media. I’m just confused cause I have no care in the world what my ex’s are doing because I’m over them. So of course it’s leading me to believe he is still not over her, misses her and wants her back. If that’s the case I know I should let him go and move on but I just want opinions because maybe I’m overreacting?
Why does he keep looking up ex?
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