[N]euroscience has indicated that the human brain simply doesn't perform well at free-associating text that, on its own, has little inherent meaning. As one of the papers cited puts it, "the multiple-password management crisis [can be viewed as] a search and retrieval problem involving human beings' long-term memory." And, although our long-term memory for images and words that we've assigned meanings to is quite good, we don't do as well with passwords, which (ideally, at least) should look like a near-random string of characters. It's another challenge entirely to remember which password to associate with a specific account.
Um, one word: 1Password.