The only demonstrable index to a man’s desires is his acts, when you’re speaking of past time: what a man did is what he wanted to do.
The idea is that you could have conflicting desires — say, the desire not to have dinner with us and the desire not to offend us. If you end by coming to dinner it’s because the second desire was stronger than the first: other things being equal, you wouldn’t want to eat with us, but other things never are equal, and actually you’d rather eat with us than insult us. So you eat with us — that’s what you finally wanted to do.
You shouldn’t say you’ll eat with us whether you want to or not; you should say you’ll eat with us if it satisfies desires in you stronger than your desire not to eat with us.