You ask, “Why?” It is because the Lord is the witness between you and the wife of your youth. You have been unfaithful to her, though she is your partner, the wife of your marriage covenant.
“The man who hates and divorces his wife,” says the Lord, the God of Israel, “does violence to the one he should protect,” says the Lord Almighty. So be on your guard, and do not be unfaithful.