Lex wants to keep Clark all to himself, Clark is his property but Clark is stupid and easily swayed by false promises so Superboy's a threat that can just swoop in and steal his friend away if they ever met.

Lex deeply sincerely believing he's protecting Clark from the evil manipulative Superboy without understanding that he's just projecting his own less than pure intent

Lex: Clark is fundamentally a good person and as such he is equally naive as he is incapable of recognizing those dangerous taking advantage of his honest optimism and inability to see others as inherently bad people, like Superboy.
Clark: How do I manipulate Lex into not killing me?

ENG