Edit: I wrote a bit more to this answer, but apparently Yahoo! Answers has a length limit. Sorry.
World War I:
The reasons for WWI are less clear than WWII, but are just as important.
The immediate event that triggered WWI was the assassination of Archduke Franz Ferdinand by Gavrillo Princip. The Archduke was destined to become the next ruler of the Astro-Hungarian empire. The Astro-Hungarian empire was a huge country that was comprised of over a dozen ethnicities. As you can easily imagine, each ethnicity wants its own independence and sovereignty; in other words, its own country.
One of these ethnicities was the Serbs. The Serbs were located right next to the border of Serbia, a country of Serbs.. So, there was a sovereign country, and right next to it were the Serbs, that belonged to the same country, but were located in a DIFFERENT country. The Serbs in Austria-Hungary wanted independence, but naturally the government opposed it, for fear of losing land and money, as well as empowering their enemy, Serbia.
When Franz Ferdinand was assassinated by Gavrillo Princip, who was from Serbia, the Astro-Hungarian Empire considered it an act of aggression by the Serbian government. They issued an ultimatum (Something which Serbia had to agree to, "or else"), which consisted of several surprising demands that would infringe on Serbia's sovereignty. Amazingly, Serbia agreed to several of them, EXCEPT for one. Despite this, Austria-Hungary refused to accept the agreement if Serbia did not agree to each point.
So they declared war. That's how WWI started. But the assassination was just an excuse for the war. The actual causes were tensions between European nations which made them THIRSTY for war. The tensions were in turn caused by the imperialistic ambitions of each European country. These ambitions meant that each country had several colonies in Asia and Africa.
But since there were so many colonies, that meant there wasn't enough land to continue the imperialism.. After all, there's only a limited amount of land in the world. Each country, especially Germany, became so powerful that they all feared each other. France was also bitter with Germany over the results of the Franco-Prussian war in 1871, in which Germany captured Alsace-Lorraine while unifying into a single country.
The causes of WWI are vast and numerous, but here's the gist of them: Austria-Hungary wanted to destroy Serbia because its existence threatened its national security. France wanted to war with Germany to win back the territory it won in the Franco-Prussian war in 1871, and give a little dose of revenge. Germany had the Schlieffen Plan, which said that in case of a war with both Russia and France, it would defeat France within a month and then move all of its troops to Russia before the Russian Army had mobilized. All of this was tied together by the Triple Entente and the Triple Alliance, which meant that in the case two countries went to war, the rest of the European countries would also go to war.
World War II:
The causes of WWII are easier to understand, as they are essentially evil vs. good.
Following WWI, the Treaty of Versailles was signed, which enforced some restrictions on Germany - mainly, that their army would remain small, so as not to start another war. It also took some of Germany's territory and formed it into independent countries, such as Poland.
All this made the German people very angry. Additionally, the economic affects caused hyperinflation - the German unit of currency, the mark, was virtually worthless. At the beginning of WWI, the dollar was worth 4.2 marks. In 1923, it was worth 4.2 trillion marks.
So the German people wanted a strong government. This led to the rise of the Nazi party, led by Adolf Hitler. Hitler vowed revenge on all those who shamed them in WWI, as well as promising to fix Germany's economy, which his part actually did. The Nazi part won a majority in the government, and in 1933 Hitler declared himself Chancellor, or dictator, of Germany.
Hitler wanted to expand Germany's territory and "create an empire that would last a thousand years." He built up his army, breaking the Treaty of Versailles, but nobody punished him for it, so he continued. He wanted a war, and eventually he got it for himself; he staged a fake attack by Poland, which he used as an excuse to invade the country. The rest of Europe, save for the Soviet Union, Italy, and some Eastern Europe countries, which were his allies, condemned his actions, and many declared war on Germany.
In Japan, WWII began in 1937 when it invaded Manchuria, a Chinese territory. Japan was a powerful country, but it had limited natural resources and was dependent on imports for its economic survival. It was allied with Germany and Italy.
So the basic causes of WWII are simply Hitler's desire to expand Germany and to win back the "honor" they had lost in WWI. He also wanted to "cleanse" the world of Jews.