Sometimes it is because the parent is too lazy to take time to be concerned about what is bothering them.Put yourself in your kid's place and try to see things through their eyes for a moment.You might think, well I'd feel like this or worse if I were them about ______.Other times the kid is a spoiled brat.Sometimes it's a little of both.
Explain to the kid why they aren't getting a shotgun for their b-day or a new mustang and tell them thats the way it is, and then tell them if they whine anymore, you will give them something to whine about.