It feels like our brains like making shortcuts like assumptions. To make analyzing every single person we meet/interact with faster or more efficient while we should actually try and experience every person individually...

I'm not sure we can turn it off, but we can monitor it. We can note our assumptions and look for further evidence before "believing" them. Also, we have to avoid becoming too fond of our judgements, assumptions, and perceptions.

One technique is to train yourself to throw away the first interpretation of an event, observation, or behaviour and look for another, less obvious one. Then look for a third.

Many factors influence which perceptions we select and how we interpret them. These factors include our self-esteem, our mood that day, past experiences with a person or location, expectations, and so on.

Also notice when other people make assumptions around you and ask them why they see it that way. You'll probably discover their assumptions are based on some past experience, not the direct experience that you talk about.