This theory might explain Stan Lee's cameos during all the Marvel films—and blow some True Believers' minds. Stan Lee's filmed a ton of cameos in Marvel Universe movies, playing an intergalactic ladies' man, a security guard, a retired military officer, a postman, a librarian, and even a strip club DJ. Tony Stark even mistook him for Larry King and Hugh Hefner in different Iron Man films. There might be a legitimate reason why Stan Lee's always popping up around MCU superheroes: he's actually Uatu the Watcher.
The Watchers are some of the oldest beings in the Marvel Universe, tasked with compiling as much info as possible on the galaxy at large, and Uatu is the Watcher keeping an eye on our particular solar system. The Watchers, as you might have guessed from their name, aren't supposed to interfere with events (though they occasionally do for the sake of avoiding catastrophes), and they usually stay in the background—which is basically where Stan Lee is found during his many MCU appearances. Marvel's Chairman Emeritus using Uatu's cosmic powers to jump from role to inconspicuous role and learning as much as he can about the superheroes of Earth, in case he ever notices something majorly out of whack? That's yet another reason to love the guy.