I know how Lex & Clark can have a falling out (re:Smallville)...

[DR.MID-NITE]

It is inevitable that Lex finds out Clarks secret. When he does he feels betrayed that Clark was not honest with him. It causes bad feelings. Also, Clark starts to see Lex becoming more and more ruthless as he fights his dad for control of Luthercorp. Although, he is angry Lex still likes Clark. But, then toward the end of the series Lex defeats his father for control and does something that forces Clark to confront him. Now, here comes the cliched part of it. Something happens a building explodes or Lex falls, etc. When he wakes up. He doesn't remember Clarks secret or most of their friendship. He still knows that he knew Clark, but nothing more. All he remembers is that a friend betrayed him and that (forshadowing) someone "super" stopped him and tried to bring him down. Thus whatever parts of Lex that was a good guy is gone and the rest is history.....

[DR. BELCH]

Lana seems to be the key. Both Clark and Lex care about her--Clark for much purer reasons, Lex for some twisted pseudo-Oedipal thing (losing his mother at a young age left a void that he seeks to fill with strong-willed women, like Lana and Lois, who eventually dump him). And Lex wouldn't be the first guy to be led astray by a woman. Meanwhile, it's evident Chloe lusts for Clark. The kiss they shared technically was under hypnosis...but like I said, you can't force someone to do something they know/believe is wrong, even in a trance...they either have to be duped or subconciously desire it to begin with. All Chloe needed was an excuse to shed her inhibitions....

[langden alger]

well..the way i see it , it's like lex said-"i want to do great things"...he's going to gain some high power.. then he's going to have to make a choice between his outer need for power and greatness and his inner need for love and friendship-his friends in smallville..i see smallville as some kind of target for some toxic catastrope kiiling lager amonts of people or something like that, which lex inadventanly has something to do with..therefore the blood is on his hands and he could have done something to probably stop it but, his lust for power stopped him...this would draw the ultimate line in the sand between clark and lex i think..

lana will def play a part in the two beginning to drift apart, but when clark's home and possibly even his parents are hurt/killed, that's what i think is gong to put in the final nail..