One of the main reasons why the DC Animated Universe is so highly revered is because it didn't shy away from tackling mature themes head on.
Everyone's familiar with Batman: The Animated Series' darker moments, or even some of the more messed up and twisted endings that have featured in its many follow-ups, but focusing on solely just the scary and macabre occasions would be to ignore just how emotionally mature these shows were. Whether it was Static Shock or Justice League Unlimited, the fine minds behind the DCAU were never reluctant to broach all kinds of themes, ranging from just suggestive innuendo, to political conspiracy, racism, betrayal, and even death.
There's also the fact that the DCAU was one of a kind. It was a shared, interconnected universe that spanned one linear timeline, and as a result fans went on to develop huge connections to certain characters. You can see the same lessons being applied in the Marvel Cinematic Universe, and as fans of that particular franchise will be the first to tell you, when you've spent so long with certain figures, the emotional payoff is always going to land twice as hard.
The DCAU was packed with plenty of heartwarming moments (look no further than "Comfort and Joy" for a helping hand of holiday cheer), but seemingly twice as many heartbreaking ones. Both are a testament to just how clever those shows were, but those sadder moments are still doubly affecting.