I've sailed with dolphins plenty of times. They've tended to lose interest when I was on a windsurfer or dinghy after a minute or so, no matter what speed I was doing. On the other hand, when I've been on a cruising cat, they can't seem to get enough. As long as you stay over 7 knots under sail they'll follow along apparently having great fun, particularly around and between the bows. I've even seen a large pod travelling in one direction hook a U-turn when they saw the boat to play for ten minutes before heading back in their original direction.

All of this is to say what everyone else is saying. They're complex creatures with sometimes non-trivial motivations.

Many animals exhibit play behavior, which has numerous benefits. The complexity of the play behavior is fairly bounded. Maybe that dolphin demonstrated an advanced form of it relative to other types of animals.

The animal section of the Play wiki is interesting, especially the cognitive theories https://en.wikipedia.org/wiki/Play_(activity)?wprov=sfti1

Dolphins don't save humans because of altruistic reasons towards our species. Their own offspring behaves similarly when drowning, and they mistake a drowning human to potentially drowning dolphin.

The same could be said about humans' behavior toward their domestic pets, which are to a large degree bred to retain the traits of infant wild animals.

Not sure what you mean. I didn't mean to undermine dolphins; they are highly intelligent species and definitely altruistic. The point is simply that they don't save a drowning human because they care about a random drowning mammal, but because they mistake it to a drowning dolphin because we both have lungs. Not sure that has anything to do with what you said.

Oh I just meant domestic pets appeal to the same parts of the human brain that human infants do, in part because humans have bred domestic pets to retain infantile traits into adulthood. For example, an adult dog retains most of the personality traits of a baby wolf, whereas an adult wolf does not.

This is somewhat analogous to your explanation of dolphins acting altruistically toward humans because we resemble their young when struggling in the water.

In the same way, it would be very hard for most of us to ignore a lost crying puppy wandering down the street.

No, it is not that simple. Humpback whales have been observed to interfere and mess up with orcas protecting gray whale calves from being killed. It was observed several times and researchers think that humpbacks just really cannot stand orcas harming other whales, read more here: https://www.smithsonianmag.com/science-nature/what-humpback-...

Humpback also have spindle (mirror) neurons in their brains, those are thought to be related to cooperation, empathy, etc. Humans have them too, but not many other species (https://www.sciencedaily.com/releases/2006/11/061127111607.h...).

It is good to not anthropomorphise, but we really should finally move away from Descartes' view on animals being simple automatons.