If there is any question as to what a guy or gal's attitude is, try this.
Extent your hand with a smile, and say these words:
"Hi my name is _______" Then, start a conversation and be a gracious host.
If you still get the cold shoulder, ask why.
I once had a jumpseater that was a little obtuse, and it was really grating on me. About halfway through the flight I had had enough and I asked If I had said something to offend him (I knew I hadn't but here was his opportunity to vent without being accused).
Turns out he was jumpseating home after receiving notice of a death in his immediate family. Here was this guy going home to bury his kid and I was worried out my feelings. Yes I felt like a jerk, but thankfully that I hadn't more direct. It's hard to know where a guy's head is, but what I do know is as professionals we have the opportunity to help each other - professionally and personally.
Lesson learned: It's not all about me.
Second lesson learned: When in doubt ask
Third lesson learned: No matter what you feel on the inside, maintain your composure on the outside (if you don't, it could come back to make you look like a jerk).