The premise is wrong. It's not that people are avoiding social interaction, or that they are anti social (at least not on our trains). It's that the seats are close together and you don't want somebody sitting and rubbing up against you for the hour plus trip. Couples traveling together will leave the middle seat open on a three seater unless they are making out the entire way, not because they are anti social, but because it's just not comfortable.

I've been riding packed commuter trains to and from Manhattan at rush hour for over a dozen years now. I've made friends on the train but that doesn't mean I want them squished up against me for the ride home. Over the years with portable media and laptops getting lighter and smaller, book readers, tablets, etc, the space required to hold and use one of those makes the empty space next to you even more valuable.

I've seen pretty much every tactic out there (and often thought it was a great subject for a book, how to keep an empty middle seat), but in the end it's never your choice, it's the choice of the person looking for a seat if they want it or not.

Personally, I would rather stand then be in the middle seat, and it's an ongoing argument if the trains need to expand capacity since there are usually empty middle seats even with people standing. It doesn't mean those people are anti social.