so glad we made it (look how far we've come, my baby)

Sometimes, Regulus would look back on those days and smile because younger him was so convinced that he was in love with James. Other times, he would look back and know that those feelings were real. He was in love with James then, and he didn’t really know where he stood now.

He hoped beyond all hope that he wasn’t still in love with him because he knew better. He knew that James would never feel the same way. Regulus was only ever going to be James’s best friend’s little brother. He was always going to be that little kid in James’s eyes. If Regulus was really in love with James this time, he was going to get his heart clobbered, and no amount of time in healer school could teach you how to put a broken heart back together.

...

Or, Regulus Black had always looked up to his older brother. In doing so, he might have fallen in love with said older brother's best friend, which didn't have to mean anything. Until, years after leaving Hogwarts, Regulus and James were back in each other's lives as professional adults. Feelings ensue.