What are the general reasons for war?

Answer

War has been used for hundreds of thousands of years as a way of attaining a certain goal through the use of force.

Main reasons for war are:

- Money and gold, kinda obvious reason, really.
-Future wars will probably be fought over drinkable water and forestry.

-edit-
Wars are also fought for more noble reasons than greed and foolish pride. (Including lumber and water)
Some wars are started in order to liberate people from slavery and to save a nation (US Civil War). They are started to stop a person from committing horrible massacre murders that would continue if not stopped (WW2 Germany). Wars are started to enforce Rule of Law on an area or nation that severely represses the people living there to inhumane levels. (Conflicts in Africa)

The important thing is that war, or justified killing, should only be used when defending your life or your family's lives or the lives of the citizens of yours or anothers country. Most other situations can be resolved by talking things out.

-edit 2-
There are things more important than defending lives. For example Freedom and liberty are worth fighting for even if it may cost you your lives or the lives of your family or the lives of other citizens. Think about the United States Revolutionary War. The citizens could have remained a British colony with all the repression that they they were enduring. They could have valued lives as more important than a revolution for freedom and liberty but they didn't. They valued those things as more important than lives.
Additionally, talking things out only works when you are talking to another person with similar personal values. i.e. honor, integrity, duty etc. If the other person is say, Adolph Hitler, who has none of the above values, then talking things out is worse than a waste of time. Its a display of weakness and a person without resolve.

=Edit 3=
History books were invented and placed in our schools to cover up the mistakes and injustices of our nation's founding fathers.

-edit4-
There are many discussions about the intent and reasons behind what many of the founding fathers of the United States of America wrote. There are many people who disagree with many parts of our US Constitution that they would like to see clarified or modified to be in with current times. However, we are where we are, and currently, the majority of US citizens choose to follow, and keep intact, the Constitution as it is. They also do not consider that document a mistake nor an injustice.