According to Songfacts, Quincy Jones remembered:

“She’s Out of My Life”, I’d been carrying around for about three years – you can feel the pain in it, you know. And I held on to it and finally something said ‘this is the right moment to give it to Michael.
And when we recorded it with Michael, I know it was an experience he’d never even thought about to sing in a song, 'cause it’s a very mature emotion. And he cried at the end of every take, you know. We recorded about – I don’t know – 8-11 takes, and every one at the end, he just cried, and I said 'hey – that’s supposed to be, leave it on there.’

Michael wrote in his 1987 autobiography Moonwalk:

“She’s Out of My Life” is about knowing that the barriers that have separated me from others are temptingly low and seemingly easy to jump over and yet they remain standing while what I really desire disappears from my sight. Tom Bahler composed a beautiful bridge, which seemed right out of an old Broadway musical. In reality, such problems are not so easily resolved and the song presents this fact, that the problem is not overcome.

Spanish Translation