Normally, when people talk about an MBTA bus, it’s to complain. It’s hot in the summer, cold in the winter, crowded, late, and infrequent. We’ve all been there. But those complaints miss something wonderful: the temporary community we form just by being together. I frequently hand over my $1.70 to ride the 71 bus between Watertown Square and Harvard Square. It’s not long. On light-traffic days, it can be less than 20 minutes, On days when cars clog the roadways, it can take half an hour.