This is a huge debate in the international relations field known as the democratic peace theory. There are plenty of explanations for it but none so convincing to be completely accepted. It could be due to the structure of democracies themselves- leaders are used to negotiation and compromise and are held accountable for their actions by the people. It may also be the nature of the developed world itself- we find trade to be much more favorable than war and prefer more peaceful options of conflict resolution. Or perhaps its a cultural issue- the Western democracies were forced into an us vs. them mentality during the Cold War and our shared culture prevents feelings of aggression and hostility.

Interestingly enough, while democracies don't declare war against other democracies, they aren't necessarily more peaceful than non-democracies. It is up for debate whether non-democracies or democracies are actually more warlike.

International trade relations mainly. By going to war you starve your economy of billions of dollars which would create a massive anti-war movement. Imagine the recent anti-war movement but with funding and support from large businesses while the war is the cause of unemployment and economic downturn.