When you look at things from Luna's perspective, she was actually kinda justified for wanting to overthrow Celestia. Imagine watching your sister do nothing productive with her magic other than raising the sun every day and watching her get all the praise. Meanwhile, you have the ability to enter the dream realm and create all kinds of illusions, but everyone just ignores you and sleeps through your beautiful night. Imagine this goes on for years, decades, centuries.
It's always Celestia Celestia Celestia. And you know for a fact you are far superior to her. If a threat were to come to Equestria, Celestia herself couldn't protect it, she's far too lazy and relaxed. You are more of a ruler than she ever will be.

To be fair, Luna wasn't in the right either for trying to overthrow her sister. But the show really tries to paint Luna in the wrong when she was justified in going against her sister. She just took it to the extreme.