A Google search of "words can't express" gives 864,000 hits containing that phrase. That's a lot of things, written in words, that words can't express!
I don't mean to be flip. (See my previous post.) But someone has some explaining to do here. Here's a shot:
"Words can't express x" --> whatever one says about x will fall short of expressing all there is to x. (So, words can perhaps partially, but not completely, express x.)
As I said before, this is why we do other things, like paint pictures, write songs, make films, and (I might add) buy people flowers, as those activities are expressive in ways that get beyond language (i.e. word-use).
At the same time, it seems like one might find these non-linguistic modes of communication equally dissatisfying, such that we are tempted to think that nothing can fully express something, as it were, "inside us." Is that thought an illusion?
(I'm thinking here, for those who know it, of Wittgenstein's remarks aimed at debunking the notion that there are "private objects" or could be a "private language"...)