Clark wanted it himself. He broke up with you, breaking your heart. Clark is tired of hurting you by hiding his secret. He saw how it hurt you that Clark was lying to you.

And so, it's like you moved on. Clark was surprised when he found out that you were dating Lex, his "best" friend. Clark didn't understand what you saw in him, because Lex was mean, rude, and treacherous. Chloe told Clark to understand your decision, because Clark broke your heart after all.

When Clark found out that Lex proposed to you, he was surprised and devastated. He loved you. So much that it hurts. Clark overheard your conversation with Chloe. Where you said that you didn't tell Lex your answer, even though if you were dating Clark, you would have said “yes” to Clark without hesitation. This gave Clark hope. He couldn't let you marry Lex.