The 46-year-old actor announced his split from wife Jennifer Aniston in February while the 25-year-old actress split with beau Justin Bieber in March.
In recent weeks 'they've become closer than ever,' says the source.
'It's turned into something more than friendship.'
Justin has been having the time of his life since he and Jen, 49, ended their two-year marriage. The actor, who loathed living in LA, moved immediately back to New York, where's he's been spotted riding his motorbike, hanging with young models and hitting the hottest parties.
'Justin felt like a caged animal with Jen, but now he has his old life back,' an insider told Life & Style. 'He was never meant for married life, and he's been all smiles since he and Jen split up.'
The source adds, 'While Justin loved Jen, they had vastly different lifestyles.'
'Now, Justin is doing exactly what he wants: living in NYC, biking around the city daily, working out and hanging out with anyone he likes,' another source allegedly spilled to Life & Style. 'He's living on his own terms. Jen only like to leave home to work or promote something.
'Justin is a free-spirited New Yorker who wants to be out and about every night of the week.'
Despite the 21-year age gap, the couple find they have a lot in common.
'Selena finds Justin a breath of fresh air,' a friend told Life & Style.