Why do we stalk the ex-gf?

Hear me out. I dated a guy for 7years. He didn’t “cheat” naman daw but we broke up so he can pursue another girl “officially”. Years later, I’m in a relationship and very happy. My ex broke up with that girl and got a new girlfriend. And yet, I still find myself randomly checking that girl’s social media profile (the girl after me). Tho mga years din ang pagitan when I check. Like what is she up to? Is she dating? What’s her job now?

Is there any psychological reason for this? I know I’m not bitter or anything and I ask myself why I’m still doing this. Lol. Like why am I not stalking his gf now? Why am I stalking the other ex? Are there any girls here that do the same?