expression
love and reason do not go together
Proverb meaning that romantic passion overrides logical thinking; people in love act irrationally.
色事は思案の外というから、彼があんなに夢中になるのも無理はない。
As the saying goes, love and reason do not go together, so it's no wonder he's so infatuated.
A traditional Japanese proverb; the exact origin is uncertain.