I tend to get uncomfortable with people who I don't particularly know well, unless I'm just in a friendly and good enough mood to get to know someone.
Among others, people in power are sometimes tough to talk to, especially if they have too much arrogance in their tone. o.o (Seriously, arrogant people make me bark...a lot. Essentially bringing out my worse side, which isn't really fun for me.)
People who simply seem condescending. I don't tolerate being talked down to no matter where I might be on the structure, it pisses me off. I can deal with authority as long as they're polite and understanding.
All of that being said, people who know me and constantly catch me on a bad day and KNOW that I'm in a bad mood are the hardest to talk to. x3 (especially if I'm uncomfortable around them for any other reason)