Once you have been in business long enough, you realize business heavily relies on trust so you could make decisions fast, and without looking over your shoulder. Being ethical attracts other ethical long-term players in the network who start doing business with you.
You want to be honest because it leaves you with a clear mind, without needing to worry what was a truth and what was a lie. Being honest will also reject people who only want to hear pretty lies, and you will remain surrounded with people who want to know you for who you are.
In negotiations, if you always try to get the best deal for yourself, you will win at first, but over the long run, people will recognize that you are not acting fairly, and they will tend to avoid you. On the other hand, if you are thinking long-term, more deals will come your way.