Superman was accustomed to the plaudits of the populace. He hoped he wasn't arrogant about it, but he did like being appreciated.
So it hurt that Conner loved Lex the most. Clark had borne and nursed and changed him, but Conner only brought things to Lex -- from his baby smiles and babbling, through his preschool drawings, right up to his inevitable straight-A report cards.
But when Conner started flying and joined the Titans, and Clark caught Lex's face set in that bone-deep fear he'd only seen the day Conner was born, he was finally able to let it go.