This is something that has chaffed me as of late. I've seen way too many assumptions being made about people around me, many of them slightly offensive, all of them completely avoidable. When we assume we loose out on wonderful opportunities. Not only that, we make ourselves look like close-minded ignorant buffoons. Within a professional community, this is a very bad thing. If you assume then you miss out on relationships that could be formed. You also tend to misinterpret the meaning of everything that the target of your assumption says, since you view it with a filter.
As we're born, we don't innately have the ability to assume. Assumptions are a survival instinct. We are taught (through experience and explicitly) to assume certain characteristics indicate "bad" people, whereas others indicate "good" people. It is difficult to not assume. Assumption is natural, we do it every day. If something looks like a duck, and quacks, then we first assume it is a duck, right? Assumptions about people are also highly related to stereotypes, you might not have personal experience that drives an assumption, but you've heard and imprinted opinions from others that drive what you think (either consciously or not). It is extremely important to realize that this is happening, it impacts our ability to communicate; both in person and in the digital world.
Modern technology facilitates fast, easy communication with people all around the world. What is missing from this medium, are the facial expressions, intonations, and other patterns that we as humans rely upon to discern meaning. Divorced from these expressions we must be particularly careful when reading messages from others so that we don't assume (infer) meaning that isn't there (especially considering that most e-mails, texts, etc. are hastily constructed). Likewise, digital social networks enable us insulate ourselves from diverse viewpoints and discourse through "block" buttons and "un-friending" of those who we find "offensive." Yes, these are useful for genuine harassment, but these tools are increasingly used for reducing divergent opinions within our information spheres. How about engaging people (with...diverse views), discover why they have those opinions, perhaps challenge them to change, or even change your own view?
In the end, we're all challenged by our "gut" reactions. These assumptions that have served so well to protect us in the past should most often be ignored, otherwise we risk missing out on potentially wonderful relationships.