Other theories: people just don't introspect much, people like being vague because "I love you" pleases someone who wants to hear "I want to work to make you happy" when you mean "I have lots of fun on dates with you", people before the advent of self-help books had a taboo against discussing (and generally expressing) feelings qua feelings and used preference-revealing actions instead.

I'm given to understand that English is unusually bad in this regard; that would seem to screen out standard human variance unless English-speakers are unusually varied.

Also, I don't think it's generalizing from one example; humans demonstrate pretty standard emotions (and facial expressions, for that matter) AFAICT.

(Also, there's The Psychological Unity of Mankind. We don't want to overgeneralize, sure, but evidence regarding one human mind is, in fact, evidence regarding all of them. It's far from overwhelming evidence, but still.)