Often the reason why cats join you in the bathroom is because through the scent of your faeces and urine you have advertised your presence and metaphorically given them a calling card and an invitation to come. The word ‘bathroom’ in America euphemistically means a toilet. You could rephrase the question, ‘Why do cats join you in the toilet?’.
You won’t normally find the answer in books. The number one Google search result from inverse.com (at the date of this post) presents the ideas of a ‘cat scientist and wildlife biologist, Mike Delgado. He thinks it is because cats like the surfaces and because we are a captive audience. He does mention the ‘smells’, that the cat finds familiar but he doesn’t expand on this.
One obvious reason is that many domestic cats follow their human caretaker around the home because they like being near you. It is reassuring. When we go upstairs to shower they sometimes follow. But, in my opinion, this is more often than not about pooping and pissing.
To be fair there are multiple reasons but, for me, the main reason why cats join us in the bathroom isn’t because we are covered in water and showering unless your cat is a Savannah or perhaps a Bengal cat because these cats sometimes like to join their owners in the shower. They are wildcat hybrids and the wild cat element comes to the fore. Water is a part of their lives because it’s where prey is found.
I am referring to using the bathroom as a toilet not as a wash place. My theory is that domestic cats become animated and excited by the smell that we produce in our urine and faeces. Urine and faeces are used as scent markers by domestic and wild cats. We know that. They advertise their presences to all others. Scent marking is a calling card. The scent from urine or faeces is an advertisement that the cat was or is there. And the purpose is for other cats to avoid the cat making the scent markers because they are strangers and potentially hostile. In this way they usually avoid physical conflict.
However, our cat will normally relate to us as an affiliate or a parent. It is normal for domestic cats to be bonded to varying degrees to their human companion. When we make this strong scent they treat it is a request to be with them. We are scent marking. Our scent is us. It is a major aspect of us to our cats. Normally cats smell our body odour in recognising us and getting reassurance from us by our presences. Going to the toilet magnifies that odour by a factor of five or more. Our cat sometimes can’t resist it.
We are advertising our presence and they want to be with us. In conclusion it is a calling card to which our cat responds positively because they want to be with us.
P.S. Obviously it does not happen all the time with all cats. There are natural variables in the friendship between human and cat and the individual characteristics of the cat. I am generalising but this is a definite form of domestic cat behaviour.
P.P.S. This is a second or third go at this post. These are my personal thoughts. If you have different ideas please tell me.