I think I have learned more playing Doki Doki Literature Club than I have in an entire school year. Even though it's a horror game... it teaches you so much.
Different types of people, each with their own opinions and problems.. It shows that even if someone seems happy, you never really know what they might be feeling.. You can't just asume that everything is fine, and that if you are mean to them that they'll be fine. Everything can hurt.. You don't know what they are going through.. You don't know them. You may have known them your whole life, and you'll never be able to understand what is going on inside them.
It also teaches you that sometimes even the smallest thing can make someone happy. Just be nice to them.. consider their opinions and feelings before you say something..
And near the end of the game, pay attention to what Monika says. She will talk to you about Sayori and how to help deal with depression