Presuppositions are the assumptions that we make about life and they form the supporting structure behind what we believe and what we communicate to others. If you listen to what people actually say then you will hear their presuppositions (what they pre-suppose) in what they say. By listening closely we can uncover their unconscious beliefs and learn a great deal about their mental model or map of the world in which they operate mentally. Of course, people do not communicate their presuppositions specifically and directly but they do include unconsciously in everything that they communicate.
The interesting thing about presuppositions is that they act covertly but we have to in part accept them, to understand what another person means by their communication with us.
Presuppositions can operate positively and negatively on our lives by limiting us or empowering us. By pre-supposing that something is or that it is going to be the way that we suppose/expect it will be, we can limit ourselves and our responses to it (life's events). Just like most beliefs, presuppositions tend to be based on what was or what happened yesterday and not on what is today. They are generalizations about the past which can limit the future.
Presuppositions are therefore not always objective and are often subjective. They are formed from our own unique experiences during life. The challenge comes when they do not serve us well, causing conflict between our conscious desires and our unconscious ones.