Guest Gerry is wrong about du and mir being nouns but is correct that Yiddish, when written in Roman characters, does not capitalize nouns; still, if the title of Sammy Cahn's ditty were written in German it would be as I stated, but that is just a random example, all titles - songs, movies, books - are so written, with only the first letter of the title and of nouns within the title capitalized.