"Because I'm the Client."

There will be multiple times in your career when the client does not go with the agency recommend. They'll choose another director. A different illustrator. The execution they feel a little more comfortable with, even though the one you're recommending is all but guaranteed to get you a Lion at Cannes and have the country tweeting about their business.
To be fair, agencies should always be prepared for this. If we don't show bad work, our clients can't buy bad work. But sometimes we have a campaign, or director or promotional partner we're just dying to work with. And the client chooses the runner up. Or the runner up to the runner up. Ideally, we'd be happy with that. Because if it's on the rail, it's for sale.
But there might be times when the agency pushes back just a little. "Really? Are you sure you want to go with that campaign? Can you tell us why?"
A good client, who sees the agency as a partner will explain themselves. Even if they're not very clear, they'll try. "I just feel more comfortable with this director because he's got a lot more experience in our category." Or, "I think this campaign will resonate better with our target market.
But when the response is, "Because I'm the client," watch out. That's not too far off from a husband telling his wife to have his supper ready when he comes home from bowling with the guys "because I'm the man in this house."
"Because I'm the client."
Those are dangerous words. Because they state the obvious, explain nothing, and are an attempt to put you in (what they perceive to be) in your place.