Why are we so unkind?

We humans can be quite unthinking. I noticed the same antisocial behaviour twice today by different people at different places - at the markets and on the walking track near my place. On both occasions, small groups of people congregated in a thoroughfare, making it difficult or impossible for others to pass. On the walking track, the four people who stopped for a chat only needed to step about three paces to the right or left, and they would have created a clearway for others. Apparently, however, that was too much trouble, so the people using the path had to make a diversion around them.