If his sister died, or he's getting paid peanuts, he still has to drive respectfully and safely to avoid causing any OTHER people on the road to die.
Also, I don't think the financial information is correct. I've spoken with truckers in the past and many are making around $40k per year (hearsay), which isn't that shabby for the lifestyle.
Lastly, welcome to life. Guess what. Work is hard, people are mean, and most jobs become frustrating if you are in them long enough, and everyone has personal problems.
I'm not saying that it's OK to flip someone off, but I understand the helplessness of being in your vehicle with your family and almost getting run off the road by some trucker who seems to be "numb" to the fact that they are driving an enormous and deadly machine 70mph down the highway.
EDIT: Not sure why I'm getting down-voted, perhaps a few rebuttals in the comments??