Why do I lose interest when someone likes me back?

It seems like I just like the chase, but I don’t know what it means.

I’ve been talking to this girl and I’ve been obsessing over her while she’s away, daydreaming of making her my girlfriend, and looking at her pics and old texts(we mainly just sext). When she started liking me back and sending me texts and tagging me in lovey dovey posts I backed away and it’s like the spark is just gone.

Does anyone know why I like people who don’t like me back and suddenly lose interest when they like me back?

I also start liking my ex’s when they leave, I’m just confused.