Not-So-Honest-Nick...

What are Nick's motivations for setting up a meeting between Mrs. Daisy Buchanan and her former lover, Gatsby? Nick is a self-professed honest man, but perhaps everyone has a bit of a dishonest side. First, Daisy is Nick's cousin. Naturally, Nick is going to be concerned for her happiness and well-being. He has met her lousy husband, so he knows what a pill he is. Not that two wrongs make a right, but Daisy's husband fools around with no consequence at all, so why should he believe she will stay faithful and true to him? What goes around comes around, as they say. Nick has even met Tom's mistress, so he can easily see what kind of Daisy is with and probably wonders why she is with him in the first place.
Second, Nick seems enthralled with the very presence of Gatsby. He seems protective of him. He seems to watch his every move and want to be in his circle of friends. When Gatsby's lights did not come on one evening, Nick rushed over to make sure he was alive and well. He also protects his secrets. He makes the effort to visit Gatsby regularly. He is deeply interested in his past, his ideas, and where he came from.
Third, Nick has another conflict of interest that would help further his decision to set up Daisy and Gatsby, the Miss Jordan Baker. When a heart is blinded by love, every decision seems to be made upon that foundation. Jordan asked Nick to set up the meeting for the two lovers, and since it was she who asked, it's easy to see why he consented. Perhaps Gatsby did that on purpose, knowing the leverage she had over the poor fellow.
As far as the "Nick-being-honest-all-the-time" thing, I don't believe it. He's definitely proud about being so, but just because he says he is doesn't make it so. Toward the beginning of the book, Nick says that Jordan is dishonest, but that it doesn't matter to him. Why would a person who is honest want to be with one who is not? That is odd. He seems to surround himself with dishonest people, and perhaps this is what has given him a false perception of what honesty and integrity is. Tom is having an affair, Daisy and Jay are having an affair, Jordan steals things from stores for fun. Nick knows about all of these things, but keeps them all under the rug. So in all actuality, Nick may not be as honest as he appears.