Since They First Met
Hatake Kakashi and Might Guy had been rivals since the day they met.
One had made a stupid comment about the other which blossomed into rivalry. Who made this comment varied depending on whom you asked. Though observation of the pair indicated that Kakashi was correct.
It had started small, who got the most bullseye's or ran the fastest. Then it grew, who could master a new jutsu, or who took out the most enemies. And then they grew up and the challenges became sillier, paper scissors rock, and thumb wrestling. Both so badly wanting to return to simpler times.
And somehow they both survived to old age, though their students said it was stubbornness rather than any thing else. Life was a competition, and neither wanted to lose.
Posted from my phone in a crummy hotel.