It's an Apple Photos feature. It can be enabled or disabled in Settings -> Apps -> Photos -> Featured Content.

It will pull together photos of a person or animal over a period of time, a set of photos from a particular date, a group from a place, or similar and make them into a slideshow.

Sometimes it comes up with good ones (I get quite a few of my daughter and my dogs) and sometimes they're hilariously bad or random/contrived.

Can't say they've ever really made me sad, but I may not be in the same stage of life as the OOP / not looking for internet joke points.