Someone recently described running as a selfish form of exercise. I understood what he meant and understood the implications for Taiji practice.
I think people reach only a certain level in Taiji because they approach practice selfishly. Group practice can counter this and can even provide practical skills. You train an awareness of your surroundings and a matching of movement and timing with the people around you.
Whether with a group or solo, Taiji is a not navel gazing practice.
(photo by Kjell Tjensvoll. Click image to see his Flickr page)