A little bit more from the novel to answer your question:
Michael: What made Sollozzo think he could get a deal with you?
Sonny: Sonny flushed and he didn't answer for a moment. Then he said, "We had a meeting a few months ago, Sollozzo came to us with a proposition on drugs. The old man turned him down. But during the meeting I shot off my mouth a little, I showed I wanted the deal. Which is absolutely the wrong thing to do; if there's one thing the old man hammered into me it's never to do a thing like that, to let other people know there's a split of opinion in the Family."