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This joke depends on the theory that commuters are so used to sharing small spaces that they will do even when they are not commuting. This strikes me as unlikely. They are however seeing a psychiatrist…
The missing text under the picture reads as follows:
It all stems from the time I discovered that The Consumers Association despises everything that I possess.
At that time, I remember The Consumers Association was the main source of unbiassed opinion about all the products that we used to buy. Nowadays the Internet provides that and much more speedily.
There is no missing text from this picture. We are forced to deduce that the dog is indicating to his owner that he regards the tree as a mere substitute for the chair.