You both had grown up together and when you came of age to choose your faction you both choose Dauntless. When time passes and the two of you move up in ranks you eventually fall out.

You barely speak to each other only for trainings for the newcomers. He never explained why he suddenly pushed you away but eventually the pain of losing him became a memory

Tobias observed you from the other end of the training room as you helped a newcomer. You felt him watching and became annoyed quickly