I suppose the statement 'people are short valuations from birth' is not precise, people are short valuations (the avg valuation) during their working years, in the other periods they are long valuations. Before birth, they are long valuations because bubbles and bull markets helps their PARENTS to be wealthier, this leads to a better quality of life early on and a larger inheritance. After they grow up and they start to work, they are short valuations because the portion of their income that is not spend (savings) needs to be invested, the lower the average valuation over their working years (the higher the average expected real returns) the better as they compound savings at a faster rate (and declines driven by mean reversion in valuations are a good thing).
At some point they will get to a point where money brings less and less benefits, if they accumulated enough, it might not even matter anymore what happens to valuations (they got enough to live comfortably even if they never produced another 1% in real gains). But to the extent that they want to donate a lot of money after death or during a period before death, they are LONG valuations because the want bull markets and bubbles to happen so they can leave a bigger amount (perhaps a larger inheritance to their children as well)
So, unless you are unborn, is far from a working age (like children), is retired already or is about to die, forget this nonsense about 'valuations are too high', its just going to hurt people to listen to that