I have a threshold for dealing with people. I may have gotten the hours down. 4.5-8 hours is my limit. Within that window I start to get really annoyed and unpersonable. I can still be a nice person, but it's really hard. Those hours don't have to be spent in the same room either. If for thirty minutes of that I'm watching tv and the other person is in my room I still am going to get a little peeved. It's nothing personal.
I get energy from the inside. I stay in and read, and play music, and write, and these things recharge me. Being out with people, no matter how much I enjoy it, is draining. That's true of everyone from my family to my best friends. I've occasionally had girlfriends who I could put up with for longer, but that's an exception to the rule.
I hate to make people feel put off or unhappy because I'm getting annoyed, and I really do try to prevent it. I just have to have a refractory period before I can start going again. I love people (well sometimes I hate them, but for the most part I love them). It's not a function of my like or dislike of someone. My favourite people annoy me after a while.
Such is Introversion. No matter how personable someone is when you meet them, or how nice they are, they still may be introverted. One may still get their energies from the quiet spaces of home. I live in my head, and I'd rather not do anything about it.
So if you know someone like me, give them a break every once in a while. If they get snappy after a few hours of time with you, it's probably not personal. Rationality is hard when you're drained.