He didn't become cold.He became careful.

Sometimes life breaks you so much that you become ruthless towards the one you love. The emotional one becomes indifferent. The one who felt everything becomes numb. That person was never like this but the world made him. And now he feels everything is transactional. Time, effort, love, he keeps everything in account because he had given everything without account. But now that same person only sees the outcome. He takes decisions not with love but with logic. He's not heartless, he's just evolved.