If a film, a book, or a person is preachy, they try too hard to push a lesson or an opinion — and the speaker does not like it. The word always complains a little. A preachy story keeps stopping to tell you how you should think. A preachy friend sounds as if they are giving a sermon at the dinner table. It comes from preach, the church-speaking word, though actually you use preachy about school talks, adverts, and parents too: anywhere the moral is too loud. It does not mean the idea is wrong. It means the way of saying it feels heavy, as if they know better than you.