Why doesn't Twilight ever think she deserves a reward for all the times she's helped save Equestria?

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Too humble for her own good.

Likely because that was how Celestia taught her. With Sunset, the issue was obvious so Celestia wanted to avoid fostering arrogance.

She might, but it declasse to say so.

She proubly sees what she is doing more as a duty than anything she does not need a reward because it was her duty to do it not because she was payed or asked to do it

I mean it's mostly cause there are other things alongside it to celebrate instead, like Luna returning instead of celebrating the defeat of nightmare moon.

She is humble. But more importantly, I think she is not used to be the center of attention, and prefers to avoid standing out.

Which is difficult when you are her.