Acharya Prashant explains that the inability to say no is fundamentally rooted in fear within the mind. This fear arises because the mind believes it has something to lose, such as a relationship, a sense of security, or the approval of others. He points out that many problems in life stem from this inability to offer resistance to external forces, situations, or the crowd. When one is unable to say no, it is because they have stakes or greed attached to a person, authority, or institution. This dependency creates a situation where one's sense of self is borrowed from the opinions and actions of others rather than being self-derived.