Are we meant to analyze jokes here? It might be too heavy a topic for New Year's Day so if it is OK, I would just like to say that, like most envy, I think the cartoon is based on fantasy.
Single or coupled, childless or parent, there are pros and cons either way so I figure it is better to concentrate on the pros of where I am at and remind myself of the cons of where I am not and don't mind if the other "team" does similar. Whatever gets us through, maintaining our dignity... and laughing.
Good old humanity. Whether single or in a relationship the other state is optimal. When you're single you don't have to deal with the issues of being in a relationship and when you're in a relationship you don't have to worry about the issues of being single. The fantasy of what we're not currently living with will always be better. It's the hedonic treadmill. My fantasy is unbelievably amazing and couldn't possibly be, but my reality is pretty cool so that's fine with me.
Would we even be here without the fantasy? The marriage, the procreation? We mock youthful optimism but think about how few great business ventures and scientific achievements would have occurred if people were realistic about the odds.
Anyone single (probably older) and wise to all of this will have a hard time finding a willing partner where the vow is modified from "I do" to a more realistic "You'll do".