Let me tell you about something I hate - smugness. You want to know something that bothers me even more? I can think of many times in my own sordid past when I have been the one committing this sin. I'm going to admit something to you that I hate to admit: right around the time when I was leaving on my mission, and soon after I got into the MTC, I had an incredible air of self-righteousness. One of my friends that was also serving a mission had a girlfriend that he left behind that I thought he was maybe too focused on, so I wrote him a letter telling him to redirect his energies away from her and back toward the mission. All that may not sound that bad, but you also don't know the feeling I put into that letter and what an incredible moron I was for thinking I had the market cornered on being a haughty 19 year old missionary who knew everything about anything. Dave and Laura can probably guess who I wrote that letter to. I've since apologized, and every time I think about it, I want to apologize again, which may even happen shortly after I finish this post.
There are different manifestations of it, but it often gets expressed in a form such as, "well so and so has (or hasn't ever) done this or that," or, "I would(not) do this or that." Usually said with arms folded, with a smile showing no teeth, and typically comes as a statement that leaves a lot of implied things that aren't actually stated. Something along the lines of, "Well I would never have done that (so you are an idiot for doing so)." With the arms folded.
It also often gets an additional clause in there that helps alert you right away that the person has no idea what he or she is talking about: Well, when I get married/have kids/etc. I will always (never) such and such. The fact remains that you are not at that point so you don't have any idea what the heck you will or won't do.
Here's what I see as the source of smugness: ignorance borne of inexperience, or some other lack of knowledge. It's making assumptions about things that you just don't know much, if anything, about. In the last month or so I have run into several prime examples of smugness, which often doubles as a general overriding feeling of douchebaggery, and I could just as well punch it right in the face. I want to hit it right in the middle of the face where it would break the bridge of the nose and would leave a forever reminder of why it ever happened in the first place. Don't fold your arms at me, and wipe that smirk off your face.
You can go to hell smugness. I want no part of you.