When you really come to like someone, you can't afford to be thinking about other people.
When you come to truly like someone, you don't have the ability to be thinking about others.
ご質問ありがとうございます。
"can't afford to~"を使っても不自然ではないです。
① "When you really come to like someone"＝「誰かを本当に好きになったら」
"you can't afford to be thinking about other people."＝「他の人たちのことを考える余裕はない。」
② "When you come to truly like someone"＝「本当に誰かを好きになったら」
"you don't have the ability to be thinking about others."＝「他人のことを考える能力など無い。」
ご参考に！
アメリカ合衆国 
