The quote often attributed to Winston Churchill goes, "You have enemies? Good. That means at least once in your life you stood for something." I like this one and used it a lot to guide my dealings with campus administrators and figures in college. It is an applied view that you can never please everyone, and that some folks could use the headache of being opposed.
We all have folks we do not care to be around. In the south, our "politeness" dictates we never let those folks catch on to that fact. In rare cases is the acrimonious relationship exposed for even the most casual observer. Instead, we play nice and bitch about the encounter later.
But do we still have true adversaries? I have been in shouting matches with folks and we could still go grab drinks later once the dust settled. But I also know people that our personalities just do not mesh, and we both would go to great pains to avoid one another. Others still that I would put in that category ten years ago have come up and hugged me or shook my hand, which leads me to believe that some feuds can have expiration dates. Mostly because it takes too much energy to stay that irritated for too long.
So perhaps adversary is as transient of a status as friend.
Workplace adversaries are the people or groups that manage to torment you without even realizing what they are doing. And it typically stems from personality conflicts rather than competition (unless you work in sales). These are best managed with a more open/honest dialogue to ensure mutual goals. In short, neither party should prevent the other from earning a paycheck.
Social adversaries rarely begin that way. Friendships can even devolve into such a state when money or attention of another are involved. While fascinating to watch, these can be particularly caustic if reputations he damaged or distorted in the process. I cannot figure out what the catalyst for these are, other than that they start with characterizations that magnify the negative (e.g. he is a self-centered jerk; she won't shut up about [blank]). It poisons social circles within days.
I can accept that some folks just have no desire to get along. At least they are being hardheaded about something they believe instead of totally passive to the whims of others.