I have noticed a lot of people make assumptions and then start believing them and telling them as facts , like what's up with that?

And another crazy thing is a lot of the time the people they tell the assumptions to just believe and accept it with no logic.
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I once asked a person who made an assumption . I asked them to explain their logic so I could understand where they where coming from .

Their response was " I just thought so"
And I responded " you just thought so ? But why "
In which they responded " I just thought so"

The crazy part was they seemed to believe this was okay and it was not a self realising moment of something perhaps that needed to be looked into .