The US rapper explained that he and Ariana took things slow and built a friendship first, with Sean even confiding in Ariana about his "problems".
Speaking in the latest issue of Complex magazine, Sean explained: "We started as friends. I'd never done that. Usually you meet a girl and then start dating. We always saw each other in the studio, and we'd talk about songs we were working on.
"It was cool to have a girl that I could talk to about my problems," he continued. "I did that with her. It's not some fake-ass, inauthentic s**t when I say we were friends. This is something special and I appreciate every moment of it."
Earlier this month, Ariana dismissed rumours that she had split from Sean by posting photos of the pair enjoying the weekend together in Lake Tahoe.
Sean, 26, and Ariana, 21, were first linked together in August after they were pictured holding hands while backstage at the MTV Video Music Awards.
Initially, the pair remained tight-lipped when grilled about their relationship, but Ariana finally confirmed their romance in October last year, describing Sean as "one of the most amazing men in the whole world".
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Ariana Grande - 'Right There' featuring Big Sean: