Humans are terrible password generators, the problem lies with the fact that thinking up of random / nonsensical and unrelated data is not natural for our sense making, natural thinking brains. The other problem is even if you do somehow think up of something seemingly random and clever it usually either isn’t random or clever enough, has been thought up before by someone else, or is easily randomly guessed by a computer because we simply cant cope with the extreme toughness of digits, characters and ASCII that is needed to beat a computer.
There are a few methods for secure password generation, the best being a completely random set of the whole alphabet, uppercase and lowercase including ASCII and symbols. However this is only good if you can store such passwords somewhere completely safe for copy + paste when logging in with such passwords. Otherwise it is completely useless.
This page offers a few methods for trying to generate a secure password, depending on how memorable you need to keep your passwords.