10 times the Israelites complained & God called it rebellion; listen to how God responded personally when we complain against Him:
How often they rebelled against Him in the wilderness
And grieved Him in the desert!
Again and again they tempted God,
And pained the Holy One of Israel.
Psalm 78:40-41
Complaining is rebelling against God
Complaining grieves God
Complaining tempts God
Complaining pains God

I believe God is trying to show us that grumbling & complaining negatively affects our relationship with Him & the community He created