When we murmur, when we complain about the trials God puts us through, we're like the Israelites in the wilderness. We make the same error they made. They had been redeemed from slavery in Egypt, they were on their way to a promised land which was also a free,undeserved gift. They had Jehovah with them, but nonetheless, they murmured and incurred the wrath of God. They were unhappy because they were unthankful.
Unthankful people are unhappy people.
That is a lesson we all need to learn and to relearn. The thankfulness begins with happiness at the remembrance of how great your sin and misery is, or was, without Christ, then remembering that Christ has redeemed you from your sins and delivered you from the flames of Hell. The more you remember these things, the more thankful you will be. And the more thankful you are, the happier you will be. Grant, O God, our heavenly Father, that we may truly meditate on all these things.