(here is an unrelated picture of some books)
Everyone thinks they know best — especially when it comes to their job, the thing that they do professionally, the job they have because (presumably) they do it better than anyone else.
And in a lot of cases, people do have jobs because they can do them better than anyone else! So they may sometimes actually know best.
This means that when you’re approaching someone in a professional capacity in the hopes of getting them to do things differently (ie., in the way that would be best for you), telling them, “I know best; you should do things my way!” may not be the best negotiating strategy. Because instead of responding (presumably as ideally desired) with something like:
‘I see! You do indeed know best! How could we have overlooked this? You are bringing a vibrant and new perspective to this issue that we have never seen before! We shall change our actions immediately!’
People are in fact more likely to respond with something like:
‘Who is this person who is accosting me to tell me that they know how to do my job better than I do? No one can possibly know how to do my job better than I do!’
The problem here is not (always) a problem of who knows best; it’s a problem of approach. If you’re trying to get someone to change the way they do things, opening by telling them that they’re making terrible choices does not tend to make them inclined to listen to you. In fact, it may instead make them think that you don’t understand their job well enough to know what you’re talking about!
So! If you’re actually endeavoring to get people to change their behavior, starting out by asking to learn more about why they do things this way — questioning instead of telling or demanding — is often a good strategy. And maybe in the process of your conversation, you’ll learn that there’s a good reason why they’re doing things the way they are.
Or maybe they’ll learn from you about an exciting different way to do their job and make a change!