Why did balance of powers led to war in Europe?

Answer 1

The answer somehow lays in the question. I strongly assume you are asking about the first World War. In that case balance of power was seeked for "prevention" of war, even though that sounds more pacifistic as it was meant beck then. Balance of power was simply seen as a way to fear your enemy from attacking you and giving yourself the necessary strength to defend yourself in case of attack and for your own attack as well. Unfortunately this constant raise in military (as well as psychology of the people) lead to some point in which further raise wasn't that easy to achieve any more, financially as well politically. The powers of Europe were prepared and even seeking for a war proving their abilities and "repaying" their costs, therefore it only took some weak reason to let the situation explode.

Answer 2

Assuming that you mean "how did it lead to Napoleon'swars with other European powers", the answer is that Napoleon's wars against Austria, Britain, Prussia and several Italian States were a consequence of the war that these countries themselves had started against France - united in the so-called Grand Coalition - with the aim of defeating the French Revolution and restoring the royal house of Bourbon on the throne of France. That revolution and the fact that France was 'exporting' its ideas and ideals to other countries was seen by these countries as a mortal threat to the status quo and the position of its rulers - and by consequence to the balance of power within Europe.