Water flows from high places to low places. That is the nature of gravity. Emotions also seem to act according to gravity. When in the presence of someone with whom you have a bond, and to whom you have entrusted your feelings, it is hard to lie and get away with it. The truth just wants to come flowing out. This is especially the case when you are trying to hide your sadness or vulnerability. It is much easier to conceal sadness from a stranger, or from someone you don’t trust.