I mean, think about it. They were "close in school" *winkwinknudgenudgesaynomore* and then they go and start up a newspaper together, and Leland is still totally in love with Kane, right? He's practically slobbering all over Kane's declaration of rights thing.
Then, later on, during the dancing scene, Leland's all jealous and stuff of the can-can dancer Kane is dancing with, and he can practically sense that Kane is pulling away from him. Then Kane goes away on vacation and comes back engaged, and Leland doesn't look happy for him at all, just kind of depressed.
After Kane loses the election, he and Leland have a separation, and Leland's drunk because it was bad enough that Kane went out and got married on him, but he also cheated on his wife with another girl too, and being numb was better than the pain. So Leland decides to go to Chicago to get away from Kane and so that he doesn't have to see Kane every day and feel bad about what he can't really have anymore.
When they see each other again for the first time for years, Leland's still jealous as fuck of Susan, and while she does suck, he's got extra vitriol for her just because. Also, Kane's hurt and angry at Leland because despite their separation, he thought their relationship had meant something, and in his hurting, he fires Leland. Then, in order to make it final, Leland shreds the check and returns the "present" Kane gave him a while back, and the breakup is official and angsty and tragic.
That's totally the story Welles was trying to tell.
(Also there was stuff in there about Rosebud and the need to be loved, whatever the fuck that was.)