@Fallenman: I thought so too, but I now think Words is getting at something pretty important.
@Words: Excuse me if I rehash something already said, but I'm getting back on track...
In general, I think the problem here is that the argument for assholishness lies on moral relativity. People like to be agreeable, and those who place more value on emotions would, in fact, say that it is better to believe in a lie than get all confused in the chance that they'd discover something new. Someone who disrupts this state of comfortable certainty without offering another comfortable place of certainty is, quite literally, attacking others' emotional well-being. Hence why such a person can be label an asshole. (This argument for moral relativity would also explain why I started seeing a problem with my own position, as I don't believe in moral relativity anymore. But that's not exactly relevant here.)
Btw, if you need me to extract more from Peirce about why certainty is desirable, I can. It is indeed quite long.
This is also why meta-conversation is important before beginning the debate, as someone may perceive the argument as a personal attack to upset them. While being contrary can create a discussion leading to truth, being contrary is also a method of expressing dislike for a person. People use agreement and similarity to connect, so deliberate dissonance is quite easily interpreted as a refusal to connect. A first debate might be seen as trying to correct someone for their own good, but a second argument in the opposite direction is easy to (mis)construe as antagonistic.
Intentionality is important, but without meta-conversation (is that even a word?) all the person has is guesses about your intentions. And they will guess, as socializing is built in with a plethora of assumptions about how we ought to behave. In other words, your intentions, unless explicitly stated, are secondary to what your behavior suggests your intentions are. Depending on the context and how you choose to express your dissenting viewpoint, you will be seen as attacking certainty, if not the competence of the person as well, and therefore emotional comfort.
I suppose then the next question would be, when is someone justified in calling someone else an asshole? Whenever that person feels unfairly attacked?