Bieber and Gomez began dating as teenagers in 2011 and spent three years in an on-and-off relationship.
They initially blamed work schedules for their first split in November 2012 and they briefly reconciled in April 2013, much to fans' delight.
But things turned more sinister when Bieber admitted that trust was the issue in the relationship.
In 2015, Bieber told ID magazine: "It's like trust and all this other stuff that starts messing with your mind.
"You're on the road. And there are beautiful women on the road. And you're just getting yourself into trouble."
On his bond with Gomez, he added: "It's like, magical. There's nothing like it. So I felt that and I just didn't want to let it go. When it was hard, I was just like, 'I want to stick this out'. So it was on again, off again, on again, off again."
Gomez's parents were said to be "furious" when the first pictures emerged of the duo seeing each other late this year.
After his split from Sofia Richie and her separation from The Weeknd, the pair began to meet up again in November.
Conversely, Bieber's mother Pattie Mallette is believed to be very on board with the reconciliation.
She lavished the Bad Liar songstress, 25, with words of praise when she spoke with People last weekend.