The phrase "reason behind the decision" refers to the cause, motive or rationale that led someone to make a particular choice or take a specific action. It is an explanation for why something was done in a certain way, rather than another. The reason can be based on various factors such as personal beliefs, practical considerations, past experiences, or advice from others. Understanding the reasoning behind a decision helps people to better understand the thought process of the person who made it and can provide insight into how similar decisions may be made in the future.