No wonder Selena Gomez is so hopelessly hung up on Justin Bieber.

The pop stars, both of whom are going through some troubled times lately (though only one of them gets in trouble with the cops like it’s his job) have quite a history.

For Selena, who went to rehab because of Justin, he was and is her first love. Emotionally and physically. He reportedly bragged about this to a pal recently:

“I knew she was in love with me when she gave me her virginity.”

A source close to the on-again, off-again couple exclusively says Bieber dished about his “first time” with Gomez, claiming the pair escaped to Palm Springs, Calif.

“It was in the summer of 2011. They flew to Palm Springs for a couple days and had a super private place to stay,” the pal said of her v-card presentation.

“The purpose of the trip was 100 percent because Selena finally said ‘yes’ to having sex with him. They agreed because they both said they were in love.”

Bieber, 19, and Gomez, 21, confirmed their relationship in February 2011, when he told Rolling Stone, “I don’t think you should have sex with anyone unless you love them.”

Those Bieber stripper photos suggest he has deviated from that mantra significantly in recent years, but the point remains: Selena was in love with him.

“Justin told me, ‘I knew she was in love with me when she gave me her virginity,’” the source dished. “He said it with no embarrassment or shyness whatsoever.”

Things went south in 2013, but the two briefly reconnected in January of this year … after which they had a falling out and Selena went to rehab for two weeks.

Gomez bailed on rehab just 14 days into a six-week program, according to a new report, failing to complete the course of treatment she signed up for.

Selena reportedly ignored the advice of medical professionals and rehab staff by leaving before finishing the program she began at The Meadows.

Despite her pulled-together, good-girl image, she had reportedly been “partying really hard” in recent months, using alcohol, weed and Ambien.

All of which can be traced back to her relationship Justin Bieber, her people say, both in terms of her fragile emotional state and exposure to his lifestyle.

Whatever drove them apart, and whomever is to blame, they may be too toxic together to make it work as a couple, although we wish both the best.

One’s been in rehab and may need to go back. The other clearly needs some kind of reality check. Here’s hoping they can get their lives together …