Remember when the very sight of your crush made your heart skip a beat? Then, after breaking up with them, one glance at your now-ex and you’d rather light the world on fire? You’re not overreacting, and it’s not just emotions — there’s actually cold, hard evidence to prove love and hate stem from the same place. In a 2008 neurobiology study, scientists scanned people looking at images of those they admired and abhorred, and found that identical areas of the brain were activated.

This isn’t surprising to Stacy Hubbard, LMFT, master trainer at the Gottman Institute, which compiled 40 years of research to create a relationship therapy method with a 90% accuracy rate in detecting why and how marriages succeed or fail.

Hubbard explains that the first part of any relationship is the honeymoon phase. Everything is hearts and rainbows, and all you can think about is that person. At this point, you have happy hormones — like oxytocin, dopamine, and serotonin — coursing through your body. “People who have higher levels of those hormones are more trusting, meaning they’re likely to miss the red flags or talk themselves out of doubt,” she says.

During the second stage of a relationship, you remove the rose-colored glasses (or complete blinders, in many cases). “The period between the honeymoon and vulnerable, trust-building phases tends to be where a lot of fighting and breakups can occur, because that oxytocin haze is wearing off and you start to notice things you didn’t before,” Hubbard says. For example, it may start to nag at you that your S.O. doesn’t make you a priority or your values don’t align.

“In the beginning of a relationship your partner makes you feel amazing, so you associate them with positivity,” says Wyatt Fisher, Psy.D, a clinical psychologist and marriage counselor. “However, if the relationship deteriorates over time, then you’ll turn it into a negative narrative.”

Adam, 26, experienced this first-hand while dating a series of guys who he thought he was in love with in the moment, but recognized the flaws of upon further consideration. “We learn to despise the people that we used to fawn over, because we are finally free of the mental gymnastics it takes to convince ourselves that someone isn’t so bad,” he says.

Lack of closure is a main reason why we may feel spite. When there’s unrepaired resentment, those painful incidents tend to linger.

According to Hubbard, this is due to the Zeigarnik effect, a psychological phenomenon that confirms we are better at recalling uncompleted tasks. So, if the breakup happens unexpectedly or without a final honest conversation, we over-analyze every argument or issue to get answers, eventually spiraling into scorn instead.

“That painful unfinished business gets stuck in your active working memory,” she says. “Whereas if you can repair it and process it in a healthy way, then that relationship is still in your mind, but you don’t have intense emotion attached to it anymore.”

On the other hand, some have used loathing as a coping mechanism to move on from a serious split. After her boyfriend abruptly ended their relationship, Rachel, 24, attempted to transfigure her adoration into apathy so she could grieve and get over him.

“There’s no justice in love, only mercy,” she says. “We never fought, and I had no animosity toward him, which proved a liability in divesting myself from the relationship.”

One of her friends advised focusing on the anger until the distress wasn’t so acute — basically, tapping into blame and betrayal as a protective measure. Then, this friend explained, she could retrieve the memories of intimacy and comfort when they no longer had the capacity to wound.

“I don’t hate him anymore, and I don’t know if I ever fully did. But this framing made me realize that my feelings obscured a lot of ways he hurt me (through carelessness or callousness),” Rachel says. “It might not have been the best method of moving forward, since it denied the recovery power of taking time to legitimately mourn a relationship, but it definitely (and counterintuitively) helped in the short term.”

A better approach, Hubbard suggests, is to reflect on the things that contributed to the failure of the relationship and what you want to do differently in the future. You could even make an objective pro/con list of what went right and wrong in order to see things more clearly.

No matter which way you handle it, letting go of hate is the hardest step. But you’ll never truly heal — or learn to love again — without it.