When we assume something negative, it shows that we might be doing that negative thing. There is a lot of power in positive thinking.
We should always have good assumptions about others not negative ones. When he have these bad thoughts about others, it displays the negativity in your own mind.
Allah tells us not to have negative suspicions. Instead, we should have positive suspicions. Thinking negative about others draws us away from our own peace and contentment.
When you think negative about others, they will think negative about you. We need to be positive. We need to think good about others. That way others will think good about us.