Assumptions help us function in a world that is always changing. Without them, we would have to constantly consider too many variables, infinite possibilities, and we would have a profileration of mental hospitals.
We assume that the vehicle in front of us will keep moving forward, otherwise we'd play touch and go with our gas pedal. That same assumption is dangerous if we assume that vehicle won't slow down, or make a sudden stop, while we punch out a quick message on our phone. In the critical few seconds in which we are distracted, our environment changes, our assumption becomes false, and we receive a jarring reminder not to rely on assumptions.
Sometimes assumptions are really excuses. We don't tell our family members that we love them, and we don't let friends know that we value them, because we assume that they know. The truth is that we are often afraid to express love and fondness, in a society where that may make us look soft. And some of us are uncomfortable saying "I love you", and other such mushy stuff. Not wanting to be smothered by mush, we save ourselves the discomfort with an assumption that our affections are understood.
Large organizations are built on assumptions. Everyone assumes everyone else knows what they should be doing. Most assume that policies were thought through thoroughly, and are the best of the options available. Some assume that the obvious flaws are obvious to those who can do something about it. Very few realize that false assumptions are crippling the organization.
If we don't question our assumptions, we become stagnant, and miss opportunities for personal growth and improvement. Some assumptions can endanger our very existence. We should always be conscious of the assumptions we have implicitly or explicitly made, and know when to reassess them.