Rufus knows how to shake hands, partly due to his father, and partly from messing around with the greeting; varying his strength and watching others do it, as well as the reactions to all of it. He's learned the same thing Squalo probably has, that a good grip shows confidence.

Of course, words do as well, and Squalo's amuse Rufus too much. He doesn't hide a smirk as he settles back in, relaxed, against the back of his seat. "Really? And what if you found someone more... interesting?"

The break indicates he could have substituted another word in there, perhaps intended to, or wants Squalo to believe he had that intention. Regardless, it's a leading question, a test of personality and intent.