There are a lot of shows that lean very hard into their whole "This is really sad" element of their plot. Shows like Clannad and Plastic Memories have become infamous for the pure destruction they waged on the emotions of their viewers. So a few writers came along and said, "Wait, if people like being sad, lets give them sad, and nothing else." And hence, Anohana was born.
The plot of the entire show is basically that there were a bunch of friends, who were not sad at all, until one of them dies, making them all very sad. One of the former friends, our MC, can now see the dead friend as a ghost, and is trying to help her pass on, by getting the old gang back together, so they can be sad as a group again.
I know it may sound like I'm laying it on a little thick here, but that is genuinely how the show plays out. Every single character can be desribed with the phrase "They are sad and also ______". One of them is "Sad but also a gamer", another is "Sad but also a big dude," another is "Sad but also likes music," and so forth. The only exception is the MC, whose phrase would have to be "Sad but also even more sad". Its like the writers sat down to write some characters, wrote SAD in big block letters, and then buggered off for lunch.
So you basically have an entire 12(-ish) episodes of people moping around in angst, unable to really communicate with other people they supposedly grew up with, and generally being angsty. I get it, their friend passed away, which is a really unfortunate thing to go through. But when you've gone through 11 episodes of everyone going "Everything sucks", it gets hard to sympathize with their plights.
The music is pretty good though. If you like sad piano music.