In a spiritual sense?
Jesus most probably said "be true". Why?
He knew about congitive dissonance, tendency for lying to grow in a feedback loop (https://www.scientificamerican.com/article/liar-liar-how-the-brain-adapts-to-telling-tall-tales/), and other things.
How did he know? He probably also wondered how he knew all this stuff and came to the conclusion something or someone told him.
There are many crazy people out there. You probably noticed. Now you know why.
