I barely know why I do things, let alone others.  By “others” I usually refer to human being-type folks... how can I begin to impute motives into a member of another species with any accuracy?  On the other hand... there is the phenomenon of “puppy play”.

My dog's name is Killer -    nobody has objected - and she's four and a half years old, no puppy by any standard I know of... and yet... there are times when I walk across the room and she'll stop... forepaws splayed in front of her, almost Sphinx-like... but with her hindquarters up in the air and her head cocked just so... her lips slightly parted and a pink sliver of tongue visible... I'd swear she is smiling…

... and I'll lift a foot... and she'll pounce on it, grabbing and tugging and pulling... and growling, a waah-waah-waah-ggrrrrruuuuuggggle kind of growl, the kind you'd expect to hear bass-booming out of the sound-system in the local multiplex in a theatre     showing a teenager's fright-movie... but it's not a werewolf tearing out the throat of an innocent passerby, it is sixteen pounds of pug, chewing on a stockinged foot, trying to get to the Secret Meaty Center of this moving treat…

       ... and sometimes she'll move along with the foot, chewing and gurgling, and sometimes she'll leave it behind because the other foot has come forward, another white, cotton sock with a Secret Meaty Center... and then back to the first... takes a bit of time to get across the living-room, let me tell you, but hey, important people are in a hurry and I'm not one of them...

 ... and then she'll latch onto the toe and ssstttrreeeeetccchhhh it out, holding her head down and her butt

up, growling... and I'll reach down and slip the sock off my foot so it snaps off and she runs with it, just a few feet away… and then she'll stop and shake-shake-shake the sock, making sure to break its neck... and then... hey, it isn't moving any more, there's no Secret Meaty    Center, it's just... dead... but look, there's still one on the other foot!... and back she runs.

And after the other sock is slipped off and killed it is time to... I dunno, challenge me; she'll assume that head-cocked-butt-up position and just look at me... and I'll take one step     towards her and she'll take off and     run-run-run-run around the room, under tables and chairs, into an open area,  circle about, two, three times…

       ... and then stop, dead cold, except for the heavy breathing... paws splayed, butt up, head cocked, tongue peeking out... and I'll take another step in her direction, or reach out with a hand... or just make any motion whatsoever, doesn't matter... and she's off again, under the table, circling in the open, dead-cold stop ...

... and this will keep up until I get tired or bored... or until, after a    particularly vigorous burst of running, she'll leap into her litter-box and go round and round and round and round, sniff a bit... and cop a doggie-squat...

... she's played so hard she's literally knocked the crap out of herself.

As mentioned before, I don't know much about my *own* species' behaviors, let alone those of another... but I'd always heard that this kind of playfulness was associated with much younger canines.  Be that as it may, it makes my heart smile to see such a   display of unaffected and utter joy... Life is Good, and It just keeps Getting Better.