From studying the causes of many wars (the Peloponnesian war, the Persian Wars, the Punic Wars, the First World War and the Second World War) I think that several coomon factors lead to war:
1) When a nations have more to gain by going to war than by maintaining peace. This sounds obvious.
2) When the status quo does not reflect power realities. For instance, Germany was defeated in World War One and the Paris Peace reflected the situation of a beaten Germany. Yet 20 years later, Germany was the most powerful nation in Europe but Germany was expected to put up with political situation that was insulting to her dignity. Similarly, Rome imposed an unfair peace on Carthage when she was being crippled by her mercenary war, but Carthage recovered and was unwilling to put up with the status quo.
3) When a nation feels that if she doesn't act, her long-term security will be put is grave danger. For example, Britain in 1914 and 1939 and arguably Germany in 1914 and Sparta in the late 330s BC.
4) Misunderstanding the goals of rival powers, e.g. Germany in 1939 did not understand that England was not willing to allow Germany to dominate continental Europe. She still held to her age old policy of standing off the continent untill there emerged a serious threat to the balance of power.
5) In more ancient times there were things called "ecumenical states" which proclaimed the right to rule the world. They could make war whenever they felt it might be beneficial. The examples are endless, virtually any attempted conquest by Persia, Rome, the Great Arab Caliphate, China, etc.
6) Sheer ambition
7) Attempts to secure long-term security by expansion or creating buffer states, eg. Rome's conquest of Italy and victory over Macedonia.
8) Ideology, e.g. the early Arab wars of expansion, other religious wars, nationalist wars and even the wars of ecumenical states.
9) Self-esteem and honour, for instance Poland's occupation of the German Polish corridor was felt as an insult to German esteem or the Athenian expedition in Ionia injured Persia' esteem. This cause is related to ideology. Injuries to a nations self-esteem and honor reduce a nations respect and hence its power, hence more reasons for satisfaction. This type of injury can also lead to the internal instability and the weakening of a leader's position.
10) Very importantly, the quality of previous peace agreements. This incorporates many of the above causes. If a peace agreement is poor, war is inevitable, because tensions will not be resolved. The classic example are the Paris Peace Treaties after world war one which vitually guaranteed a renewal of war.
11) Gambling. A daring leader may gamble by bluff, like Hitler in the 30s. If his bluff is called, it can be too injurious to self-esteem and honour to back down, as with Hitler in 1939.
I could go on for a long time, but these are some of the most important. Ulitimately, if you're a determinist, you'll believe that every war is caused by the Big Bang and that any other type of analysis is hence a waste of time.