How have assumptions played a role in the formation and development of your relationships with others?

No, I’m not asking because of the interaction I experienced in a previous thread. So, please don’t assume (ha ha) I’m speaking about you (whoever you are).

This question pertains to any type of relationship—significant or otherwise, short-term or long-term. Assumptions come in all shapes and sizes. Some are never discussed, but end up festering and affecting throughout the course of the relationship. Although, as the expression goes, to assume is to make an ass out of you and me, some of us continue to make unconfirmed speculations, theories, and conclusions about others without validating our perceptions with the ones we have perceived.

Whatever role (if any) assumptions have played in the relationship formation and/or development process, how did it affect the person making the assumption and the person being assumed? Once the assumption (if it existed) was confirmed or denied, how did the realization change perceptions and/or affect the nature of the relationship?