Here is a fantastic piece on 7 Evils of a Grumbling Spirit.

Here they are in bullet points, but you can read the whole thing here.

1. It models Satan.
2. It is contrary to who you are.
3. It is the opposite of prayer.
4. It is simply a waste of time.
5. It swallows up the blessing of mercy before it arrives.
6. It worsens sufferings and afflictions.
7. It wears the hopeless costume of pessimism.

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