Or perhaps it should in particular be "Why the bride- or groom-to-be is always right". This is not some catch-phrase built to avoid issues or make sales—it has a reason. They are always right because they are spending their hard-earned money on something that is important to them. You may believe that it is important to you, but if you are putting it out there to be sold, hopefully you are doing so because you want others to love it. And if you want them to love it, you should expect anxiety over it.
Now, what I'm talking about is a customer of a product. That is not the same thing as a "client". A client, whether of a law firm or design firm or any kind of what-have-you firm, they can indeed be wrong. They are not bringing their expertise to the table, they are hiring you because they want YOU to tell them what to love. Clients can be wrong. And you should find either the kindest way to explain it to them, or not work with them.
But customers, they are always right. It is important for me, for any of us in the mercantile world, to remember. We've already made our design and put it out there into the world, for others to appreciate. And weddings are an incredibly stressful time, and everything one buys is suppose to symbolize, to positively REEK of love. How can one stand straight underneath that weight?
So, dear anyone, have pity for customers. Have pity for all of us who must bear the burden of anyone thinking we're crazy during wedding planning time. When we fall in love with something we may behave wrongly but our hearts are right.