Is there such thing as "too sad of an ending?" the novel I'm writing ends on a fairly depressing note, not at all happy, and I'm kind of worried that that will make people so angry that they wouldn't be willing to pick up any other book written by me (beca

A sad ending is too sad when it stops being sad because of excessive sadness.

It’s similar to the When Every Character Has a Sob Story, No One Does effect. The reader becomes desensitized to tragedy because there’s just so much of it. It becomes commonplace. There’s nothing unexpected or emotional about it.

As for endings that are just plain depressing, whether the reader reacts positively or negatively depends on how you write the ending. This is where beta readers can help.

An example of a poorly written sad ending would be if you suddenly killed a major character for the purpose of ending a plot you didn’t otherwise know how to handle and/or to shock the readers. This comes off as a cop out and it can be unsatisfying.