Prax, they had selective memory.
Numbers 11:5 states:
We remember the fish, which we did eat in Egypt freely; the cucumbers, and the melons, and the leeks, and the onions, and the garlick.
They didn't rememember the beatings and how they had to work as slaves. They longed for what they knew and forgot the abuse and the mistreatment in Egypt. Kind of like the victim fondly clinging to the abuser. Thus, they longed to return to what they had called home.
They feared the unknown. Even though they had been promised a "land flowing with milk and honey", it was an unknown land. They staggered at the promises of God, in a sense, not fully trusting Him and where He was leading.
The Old Testament is our school master. What happened to them literally is happening to us in the spiritual sense. The Children of Israel didn't like the pain. They didn't like the suffering. Neither do we, so we grumble and wonder around in spiritual circles. Life seems like a nefarious riddle at times - until we look to Him for our answers.
Yes, I am thankful for trials. What kind of wimps would all of us be if we never had to be tried? As far as complaining and murmurring, I think God knows it's inevitable. Look at some of the greatest prayers of the Bible. Are they not questionings or maybe semi-complaints? David in the Psalms...Habbakuk...Job...even Jesus in the Garden of Gethsemane struggled in His flesh. I think God likes for us to question Him. He made us for communication. ...How boring would it be never to question anyone in conversation!...He knows we are going to find Him when we start looking for answers. We are turning our attention to Him and we know that we are not sufficient within ourselves.
God puts the test before us. We can tear it up and throw the paper in the trash. ...OR we can attempt to respond within ourselves and fail...If we lean to our own understanding the equation won't balance....OR we can put it in His hands, ask His help, seek His will, and knock until the answer is given. I like that expression "put wings upon your problems and they become a prayer." Having the mind of Christ is the only way to truly solve the test. Relying on "our mind", solely will lead to failure.