It's a drug, it effects your behaviour/mood, you must have done some weird shit on it
filtered coffee has had much more time to steep in all the caffeine
True, but I think it's more complicated than that. Filtered coffee is typically a coarse grind, espresso is fine---and fine grind has a larger surface area to volume ratio, which helps with extraction.
I have also heard that caffeine leaches out fairly quickly, so it gets to be diminishing returns pretty quickly.