Let's be honest, this day in age good deeds are few and far between. Most people care far too much about their own lives to insert themselves in the lives of others. That goes for me too. I am not out there grabbing every cat from a tree or escorting every senior citizen across the road. As much as I would love to pay it forward, time is money, and there is never enough in the day to be wasting it on people I don't know. Now, that being said, there are circumstances where if you don't help someone out then it is a reflection on how good, or bad, of a person you are. It may be opening up a door for someone in a wheelchair or picking something up for someone that didn't realize that they dropped it. Those are moments where it is so blatantly obviously that the right thing to do is help out that you would really be an asshole if you didn't.
Putting a hub cap on a random stranger's car may not fall into the "I have to do something here" category, but it's pretty close. Especially when the guy who did it has a full set of the SAME EXACT hubcaps more than likely sitting in his trunk. It's really all about awareness. The chances of this guy noticing that he could potentially help out are so remote that once he did he had no choice but to take action. I am sure he didn't necessarily want to bust out his spare hubcap and pop it on random car, but he felt obligated to. That doesn't mean he doesn't get potential heaven points, and self satisfaction, for doing so, but it does mean that the unlikelihood of the situation kind of forced his hand. The chances of him being able to help were so microscopic that it almost has to be a test. I'm not a religious person, but this has "God works in mysterious ways" written all fucking over it. If you can unload a hubcap, while making someone's car more aesthetically bearable, all while getting yourself closer to an eternal bliss (that may or may not exist), then you have to do it and you might as well throw in a 'Pimp My Ride' reference when you do.
P.S. The dickhead in me thinks this guy should have just put the hubcap on and not left a note. Done the guy a favor, and sent him into a never-ending state of confusion.