Christians who complain do not take God at His Word.
People love to complain about what has happened to them.
They complain about a lack of money.
They complain about being misunderstood.
They complain about the church. About not being treated well enough.
Some love to complain about their families or how they were raised.
Lots of folks complain about the weather and about their own physical imperfections.
In fact, it is rare to meet someone who can relate something negative without whining about it.
All of these things have been addressed in God's Word. We address things in prayer and we see tremendous change. But many blessings come in rumpled packaging and can only be recognized with the eyes of faith.
Soldier, listen to yourself. This does not mean one must never talk about things that are wrong or out of whack.
It does mean that your mouth should be His mouth.