Some people are just selfish, but I can see reasonable reasons behind the refusal.
I personally wash my hands at the first chance I get after getting off an airplane because the surfaces are so contaminated by dirt, grime, DNA, and micro-organisms. Airplanes, and the people on them, are generally gross.
I don't want a stranger handling my pen - I don't know where your hands have been! It isn't your seat neighbor's responsibility to have a pen for you. Even though it would have been nice of him to loan the pen, he is within his rights to be selfish and I'm sure you found plenty of pens in the CBP area at your arrival airport and went about your day with minimal delay anyway.