As human beings, we have the right to choose. Unfortunately, some people take away this right for selfish reasons or cultural beliefs. An extreme example is slavery and forced marriages. More common abuses are lying and cheating.
Imagine meeting someone new and taking a personal interest in them. During preliminary conversations, this person does not reveal that they are in a committed relationship. As time goes on, you develop a physical and emotional connection with this person and later discover they are actively involved in another relationship.
While it may be easy to carry out our lives in an oblivious manner, others should not suffer from selfish choices made on their behalf and without their input. Honesty gives others freedom of choice.