When we make assumptions about others, we are actually making judgements about them. It may not necessarily be a bad judgment, but it is a judgment nonetheless. Of course, Jesus had something to say about that didn’t he? Jesus said, “Judge not, that you be not judged. For with the judgment you pronounce you will be judged, and with the measure you use, it will be measured to you”
Basically the Lord is saying, “Caution: judge at your own risk.” Because we can get inside other people's heads, or hearts, His words are meant to give us serious pause, and to examine ourselves before making assumptions about others.
Take Away: It is always safe to assume that it is never safe to assume anything.