Silly used to mean blessed. In Old English it was the word for lucky, and over the centuries it slid down through innocent, then helpless, then pitiable, until by the late 1500s it meant foolish, and stayed there. Nice came up the same stairs the other way: it arrived from Latin meaning ignorant, spent a long time meaning fussy, and finally settled on pleasant. For a few decades around Shakespeare you could call someone nice or silly and mean much the same thing, which I like to think of as the moment they passed each other and neither said hello.