A term coined by political philosopher Adam Smith in his Theory of Moral Sentiments (or, at least in the 2013 "hip" translation of it). This phrase is what the spectator says to the person principally concerned (PCP) when there is no propriety in their level of emotion in a particular situation. It's the spectator's attempt to close the gap between their feelings and the feelings of the PCP so that they can empathize.
"Getting on someone's level" is a beautiful phenomenon that involves self-moderation and therefore gives us morality. As Adam Smith would say, we're all just a bunch of social animals.
Person principally concern: "Oh my gosh I'm so upset Professor Cullen gave me an A- in his Search course!"
Spectator: "Dude, chill. Get on my level. It's not that big of a deal."