We're legitimately curious. He is a racer, after all.
To some extent, Denny represents the everyman. He's the average guy who has a job at a local business and raises an average family for which he is a caregiver and a provider. He's likeable without being showy, and successful without being obnoxious. In fact, there's nothing even remarkable about the way Denny looks, even though Enzo does describe what he sees when he meets Denny for the first time as a fairly good-looking guy: "He was slender, with long, lean muscles. Not a large man, but assertive. He had keen, icy blue eyes. His choppy hair and short, scruffy beard were wiry, like an Irish terrier" (2.6).
Enzo calls this first look at Denny "my first glimpse at the rest of my life" (2.5), and he means that quite literally, because from this moment on, his entire world will revolve around and depend upon this man who picked him up out of the basket of his siblings.
It's almost like the scene in Toy Story when that one three-eyed alien toy gets chosen by the claw.
We can tell from Enzo's description that Denny's not an alpha male, big and blustering and domineering, though he's not meek and mild, either. He's controlled, smart, decisive, and thoughtful, and he pays attention. It makes a lot of sense that he's a racecar driver, because you need all of these skills to do a sport like that.
Once we meet Denny, we learn a lot about him. We learn that he's a semi-professional racecar driver, and that he loves racing. He shows Enzo old videos of past races and bestows upon him lots of wisdom about the art of racing (in the rain or not).
Now, we don't know a lot about Denny's life before Enzo. That's because Enzo doesn't meet Denny as a boy—he meets Denny as an adult, with a job and an already established love of racing. This very developed love of racing is thus a key factor that shapes Enzo's understanding of Denny and his knowledge of Denny's ability to control his life and his ultimate destination. Denny lives by the mantra "your car goes where your eyes go" (7.1).
Denny has some rotten luck. Seriously, did this guy break a couple hundred mirrors or something before we met him? Is that why he has all of these bad things happen to him? Sure, life can give you lemons sometimes, but Denny cleaned out the entire lemon stand at the farmer's market.
First, Eve dies. Then his in-laws sue him for custody of his daughter. Then he gets charged with raping a child. Then he can't afford his lawyers. Then—well, we digress. Tl;dr: things get worse for Denny before they get better. But like any racer knows, "No race has ever been won in the first corner" (35.42).
It's Denny's mentality as a racecar driver that keeps him from giving up—and there are so many moments where he could have given up, and no one would have blamed him. Denny isn't one to quit just because the road ahead is difficult, or because the weather gets bad. He's going to race in the rain, darn it, and pass the finish line with his head held high.
Hey, maybe that's where the title comes from. Ba-dum-ching.