The former Disney star entered the Dawn at The Meadows recovery centre in Arizona on 5 January – but not for a substance abuse problem.
Instead, it was the 21-year-old's parents who sent their daughter to the rehab facility, because they were worried about her level of partying.
"Her friends and family became increasingly worried that there might be a serious problem with Selena," a source told RadarOnline.
"Particularly, her parents. The partying was, at times, wild. They wanted to help her – and they very much encouraged her to go to rehab. Others very close to her also urged her to seek professional help."
They added that "Selena had worked in Hollywood non-stop for six years" and was "exhausted". But, they said, she "was smart enough to understand that she needed to take control of her life.
"It just got to a point where she needed to get out of Los Angeles, get away from 'the scene' and the pressures of Hollywood, to also get away from those who had, admittedly, become a bad influence on her.
"Selena needed to take the opportunity to work on herself. She realised she needed help," said the source.
The news comes shortly after the Come & Get It singer cancelled her Australia tour in December to "spend time" on herself.
She said in a statement at the time: "My fans are so important to me and I would never want to disappoint them.
"But it has become clear to me and those close to me that after many years of putting my work first, I need to spend some time on myself in order to be the best person I can be."