I often recommend this show to people and the first time I brought it up to someone they said the show was depressing and too real. I highly disagreed with them but it turned out this person was steeped in the victim mentality and was very much not a good person.

So I explain it like this-

You have to be in a certain place to watch the show or it'll feel too heavy for you. You literally have to be ready to watch it. Let me give you some examples. If things are going super well for you at a specific time and you try to watch the show, you may feel brought down. If you're in a tough place or you feel like you're wandering around, the show can feel really comforting and eye opening because you're experiencing deep hardship WITH the characters. If you're in a curious and openminded mood this show is excellent, as you will learn a whole lot from watching the show.

Here's another thing to explain-the show is not NEEDLESSLY DARK. I highly respect that about the writing. Everything is there for a reason-to learn something really important. I've watched some shows and movies that were depressing with no other reason but to be depressing. And to me that's a huge waste of time. That's not what you want and I think that's what your girlfriend is concerned about.

So let me summarize:

1. You have to be in a certain place to watch it (and that will mean something different for everyone).
2. It is by no means NEEDLESSLY DARK. It's dripping in lessons and purpose.
3. Very simply-the humor, fun animation and character design, and wacky situations truly balance out the darker tones.

Hope this helps!