Blood
In the Sohma family, relationships of friendship were often closer than those of blood.
So it was between Kazuma and Makoto. Not brothers or first cousins, but close nonetheless. From childhood through high school, they were inseparable.
But they did become estranged after Makoto went to college and Kazuma studied martial arts.
When Makoto married, the gulf between them truly became apparent. Single and married people just didn't have much in common.
They still talked from time to time, however. Like the time Makoto told him his wife was pregnant… if it was a boy, they would name him "Kyo."
Author's note: Another "best friends" story. I sometimes think that there had to be more reasons that just Kazuma's grandfather that Kazuma took Kyo in: some connection beyond that. Here's one possibility.
Sorry for it being another serious fic. I'll try to make the next one funny.