Because people only share happy moments on Facebook. When you're catching up with someone, you're probably in a "normal" state of mind. They'll share a happy moment and the mood will spike up, then drop to baseline. Then you'll share a sad moment and the mood will spike down, return to baseline, etc. Facebook is always happy, never normal, never sad (unless it's in some way a humblebrag. "Ugh had to put in 20 hours of overtime at my killer job"). Imagine living in the crest of a manic depressive's life, forever. That's Facebook.