Scientists at Caltech have discovered that there are two systems of neurons that influence whether and at what time one is either social or antisocial. Specifically, the antisocial system induces self-grooming, or repetitive behaviors.
Each system inhibits the other, so that one switches from social behaviors to antisocial behaviors. Certainly we see most people switching between these two behaviors. However, people with autism seem to have the social system turned off most of the time.
As it turns out, the social system is also an inhibitory system. It inhibits neural activity. The antisocial system is an excitatory system. It increases neural activity. In other words, this discovery supports the Intense World Theory (IWT) of autism.
The IWT says excitatory neurons are working more strongly than are the inhibitory neurons. That is, positive feedback dominates. In very social people, inhibitory neurons dominate, meaning negative feedback, meaning equilibrium dominates.
Of course, these are likely not the only inhibitory and excitatory systems in the brain. And it is likely that there will be not only other alternating systems, but also co-dominant systems. But this research provides some pretty strong evidence for why it is that excitatory dominance would result in autism.