The problem I have is that, sometimes, some people take my disagreement with them as a personal insult or an attack on them personally. E.g. the other day on a whatsapp group chat, one of my friends posted a link that said that house prices were about to fall. I said that my advice was to ignore average house prices and just work out whether the house you're interested in is worth the money; if it is then it doesn't matter what other houses are selling for, because yours will be worth it. Anyway she took offence to this and thought I was saying that she should shut up and not post the link. She started to get defensive and while I tried to diffuse it it was no good, I was just making it worse. Eventually she said "sorry I won't post it next time! I just thought it was interesting!" I said, "don't apologise, it is interesting, that's why I was discussing it!" If it wasn't interesting, I'd have just ignored it.
Anyway I don't know whether this was my fault or hers or no-one's, just a difference in personalities. But this is the way that some people view things: if you don't agree with them, then you are attacking them in some way. That's not the way I view things: if you don't agree with them, you discuss it until you have reached a mutual understanding of each others' positions. You might still disagree with them, but you'll learn something.
I find it easy these days to separate a disagreement from a full throated argument. The problem I have is with the mentality that opposing opinions are personally offensive. I used to be like that, as a look at my posting history from 5 years ago will show. So yeah, I like getting along with people, and I like discussing big ideas, issues, politics, science, religion, football, etc etc. Problem is, other people take disagreements personally, as I used to, and that puts the two into conflict. (I mean, the number of times I have to explain that I'm not even disagreeing with them is unreal!)
Going back to what aelf and warpus were talking about, I find that some friends I can argue with til we're blue in the face and still be best mates afterwards. Friends that I've known since school (so, like, since we were 7) are easy to argue with, because after 20 years of friendship, we've had so many arguments that one more (even a whopper, as inevitably happens when we go travelling together) isn't going to matter. We'll still be friends no matter what. But it's not just old friends - it's the personality of the person in question. There are people I've known an equally long time, some of whom I can discuss big ideas with, some of whom I have to avoid disagreements at all cost, lest they get pissed off and sour faced for the rest of the evening.
So I tend to have to work out who I can discuss things with. These people are typically my closest friends; perhaps their pre-existing closeness means we can have arguments without taking it personally; or perhaps their ability to not take things personally is why we're such good friends. Either way, it's something I value rather highly in a friendship.