When dealing with software features and changes that affect a billion users, it's very easy to overlook some of the implications of a proposed change or not adequately take into account peoples' emotional well-being. Facebook's Year In Review feature was supposed to provide people with some fun and light-hearted memories by showing them some of their most liked photos from 2014. In theory, this is a great idea that helps many people out and also gives Facebook some extra page views while they're at it which would be a win-win for everybody. Unfortunately, practice is sometimes different than theory. While the feature worked fine for most people and probably made many people smile, it also brought up bad memories for a lot of people by showing them lots of pictures of a former partner when they're feeling low and vulnerable. And one can't speak of the harsh pain caused by incidents such as a parent feeling hurt by unexpectedly seeing images of their tragically lost child.
Dealing with a user-base as massive as Facebook's on such an intimate level, even if an extremely small percentage of users dislikes a change or a new feature, is bound to lead to many thousands of people being upset when Facebook doesn't get something perfectly right. Facebook has always predictably been criticized for various UI changes and layout refreshes as their user-base is mainstream rather than technical, but incidents like their heavily criticized psychology experiment along with things like this indicates that they still have a ways to go in how they develop the platform while taking into account the well-being and mental-health needs of their userbase. Truthfully, this is not a trivial problem to solve and many tradeoffs are needed when designing any feature. But this is something that Facebook needs to start getting right more often