I think it's just Positive Reinforcement, right? You are given something for your behaviour, so every time you get it you are rewarded.

The Karma itself acts like a social score. You want it higher because you perceive it as valuable. It represents social acceptance and general value in the society it's from (Reddit, Saidit, etc). Someone agrees with you, thinks you're funny, thinks you're the cool kind of mean, so they give you a social point. Each time you get one your brain connects that to your social status and your Positive Reinforcement is activated... Which, I think, leads to Operant Conditioning. Although we, on an intellectual level, understand that nobody will likely ever look at our Karma and people will just judge us based on the post we make in the moment, there's a part of us that feels rewarded by the social currency we've gained, meaning we are socially valuable.

Me? I'm saving up my Karma so that, when the machines take over, I can prove my value numerically to them.