Everyone has an ex they still think about. Maybe it’s a dream, maybe it’s a song, maybe it’s just a random Wednesday afternoon. Your brain has some explaining to do.

According to a Columbia University study, pretty much everyone thinks about an ex at some point, with women reporting more frequent and emotionally vivid thoughts. Oh, and 60% of married people admit an ex crosses their mind more than it probably should. So there’s that.

According to Hannah Jackson-McCamley, a London-based psychotherapist and member of the British Association for Counseling and Psychotherapy, the brain is largely to blame. “When we fall for someone, our brains build strong neural pathways around that person and the habits we develop while we’re with them,” she says. “These pathways are incredibly powerful, and our bodies release feel-good hormones like dopamine, serotonin, and oxytocin. We then associate that person with pleasure and will continue to, if we don’t break the neural fingerprints left on our minds.”

Your brain got attached, and it didn’t ask your permission to stay that way.

What the fantasies are actually telling you

Thinking about an ex doesn’t automatically mean your current relationship is in trouble. Fixating on one can be a way of avoiding something unresolved in your present life, whether that’s low self-esteem, sexual dissatisfaction, or a general sense that something’s missing.

If an ex keeps showing up during sex, Jackson-McCamley says not to spiral. “People get stuck in the shame of those thoughts, but what you fantasize about sexually doesn’t particularly mean what you want in real life.” Sometimes it’s the dynamic you’re missing, or a specific kink, or just a version of yourself you haven’t thought about in a while.

The first love problem

First loves hit differently neurologically. The part of the brain where early loving feelings develop sits closer to our base impulses. “If we feel unfulfilled in our current situation, there might be something in our neural pathway that’s been developed in that early relationship that we long for again,” says Jackson-McCamley. The catch is that memory edits out the bad parts. “We’re never going to be 19, young and in love again. When we just remember the fun stuff, we can forget what chipped away at the relationship over time.”

The one that got away

This is the one that keeps people up at night. “If only I’d done this, that might have happened” keeps people stuck in self-blame and sliding-door fantasies. Jackson-McCamley has a good one here: “Past relationships don’t have to be a failure because they didn’t last. They can be seen for what they were, and we can use that information to better our current situations, rather than blow them up.”

When to actually do something about it

If the thoughts are persistent and affecting your relationship, think about what void the fantasy is filling and whether you can address it directly. Consider what you want sexually and whether you’re communicating that to your partner. And if your ex is still in your Instagram following and their birthday is still in your calendar, Jackson-McCamley gives it to us straight. “Reminders don’t help you to heal.”