God had provided manna for them every day, yet still they complained. They wanted meat. At first, I thought, why are they complaining; they have meat! They have a variety of flocks and herds, and God's instructions regarding the fellowship meals includes provision for them to share the meat that is offered.
Moses makes it clear that there was not enough for everyone, even if they slaughtered all their flocks and herds. So, presumably, the ones who were complaining were those who didn't have flocks and herds from which they could eat. They were complaining not because they were starving, but because they wanted more. More than what God had provided.
Americans have made a cult out of wanting more. Whether it is portion-size at a restaurant, power in our car engines, or square feet in our homes, we want more of everything. And we want it now.
It wasn't wrong to want meat. But it is wrong to look on the abundance God has provided and say, "this is not enough."
Notice that it does not say they went before the Lord and prayed that God would provide them with meat to go with their manna. I wonder if that would have ended better for them.