Why do fish like being stroked?

The aquarium at the Cherbourg submarine museum (highly recommended; wonderful museum to visit, especially with small people) has a petting tank. Where small (and not-so-small) children can stroke the backs of friendly rays. It's a large (but not very deep) tank that's only accessible to said small people on one side, so the rays have to actively swim to the people to be stroked.

Cats, dogs, humans and other furry things like to be stroked, and even the act of stroking can reduce down a stressed-out heartrate, and it makes sense that animals that rely partly on contact senses should benefit from using those senses to communicate maternal presence, safety etc. But then we get to the hairless creatures that also enjoy a tickle behind the ears, including toads (who are also partial to catfood, but that's another story), snakes, and fish, and my question for the day has to be "what do the fish get out of it?".

so, starting with which types of fish and why. Georgia aquarium has bonnethead sharks and cownose rays. Other aquaria mainly feature rays and sharks, with the occasional catfish (and starfish, crab or sea urchins to feel rather than stroke). And Nausicaa claims to have cuddly skates. Now it may be that these fish are easy to put small hands on without causing any damage from fins etc; rays, in particular offer a nice flat back to stroke with the minimum amount of coordination (sometimes a difficult thing when you're 3); maybe it's because these are naturally placid fish, and therefore unlikely to bite anyone (although it does occasionally go wrong, and in the case of the cownose ray, it's because they prefer swimming at the top of the tank rather than lurking at the bottom of the tank like most other types of ray.

Okay, so maybe the fish type is a red herring. But still, the question remains: why do the fish like being stroked? What evolutionary reason could there possibly be for this? OR maybe it's learnt; the touch tank at the Tampa Bay stadium sells ray food to its visitors. Maybe what we have here is simply Pavlov's fish.

Meanwhile, an intriguing nugget: the underwater world at SF's Pier 39 rotates its petting tank inhabitants out of the tank every 3 days because they get "tired out" in the tank. How? Why? The journey continues... To where I find that stingrays in petting tanks have their spines snipped off with pliers so they can't hurt the people stroking them. It's a good thing for us (but not necessarily for them) that they have a reputation for being docile.