Why do Enfield and Utterson go for a walk together every Sunday?

Utterson and Enfield's Sunday walks allow them respite from the tedium of boredom and business. They are pleasure walks... a chance to get out in the fresh air, run across acquaintances, and simply enjoy the day.

It was reported by those who encountered them in their Sunday walks, that they said nothing, looked singularly dull and would hail with obvious relief the appearance of a friend. For all that, the two men put the greatest store by these excursions, counted them the chief jewel of each week, and not only set aside occasions of pleasure, but even resisted the calls of business, that they might enjoy them uninterrupted.

It seems odd they are so close because they have little in common and their conversation is limited.

It was a nut to crack for many, what these two could see in each other, or what subject they could find in common. It was reported by those who encountered them in their Sunday walks, that they said nothing, looked singularly dull and would hail with obvious relief the appearance of a friend.