Assumptions come from our brain trying to make patterns based on our previous experiences. We make them about ourselves and about others. As long as we're always open to confronting them, they can be helpful. It's when we close ourselves off to seeing things from a different perspective that assumptions can trip us up and blind us to possibility.
One of the most fundamental assumptions people make is whether things are scarce or abundant. This assumption can colour everything and can be the difference in leaping forward and doing something that matters or continuing to put road blocks in your way.