well here are a few things. i wouldnt be mad at what your mad at personally, though i see weed differently i bet.

the guy with cancer is the one you were helping right? well if im not mistaken by your other post your interested in helping people like that if not him by growing MJ, even if your not growing now yourself. And that guy has probably smoke with the other guy before, and is cool with his smoking. And i dont know how well you know your neighbor, but if he knows you smoke, he may have mentioned it to the other people and they may think your cool with the fact that he did.

second, i agree, his not offering to smoke with the rest of the people including yourself is kinda selfish, but that gives you the right to do one of two things, smoke yourself should you want to, or ask to take a hit, though you would be expected to pack a bowl if you shared it with this dude.

and i dont know how well you know this dude, but what if he uses it for pain, and was feeling sore after moving trees, maybe he was medicating himself. I dont know.

Im not saying you shouldn't be upset, because i know if i saw someone smoking infront of me just so nonchalantly and atleast not even offer me a hit i would be pissed, and i would feel similar to you, but after calming down, its always acceptable to realize maybe it wasnt selfish.