In order to answer the first part of your question, it is important to stress that the Trojan War took place in the Bronze Age, which means that no true, verifiable historical accounts exist. According to legend, however, the war was started by the fact that Paris, the son of the Trojan king Priam, had abducted Helen of Sparta. According to the mythological story, you could argue that Helen wasn't actually abducted by force. Instead, Helen left Sparta voluntarily, given that the Greek God Eros had shot her with his arrow, which prompted her to fall in love with Paris. However, Helen, said to be the most beautiful women at the time, had been married to Menelaos, the King of Sparta. In order to get his wife back, Menelaos convinced his brother Agamemnon, the King of Mycenae, to join him in an attack against Troy. This resulted in the Trojan War, which means you could argue that the Trojan War was mainly caused by personal interest, namely Menelaos's desire to be reunited with his wife.
With regard to the second part of your question, it is therefore very clear that the wars of today do not take place for the same reasons. Nowadays, wars are fought for many different reasons, which are not just merely motivated by a leader's personal interests. For example, you could mention that one of these reasons for war could be the fact that a country is attacked by another country, so it needs to defend itself in order to remain unoccupied. Another reason for war might be financial gain: a country might want to occupy another country in order to gain access to their resources.
A further cause of war between countries could be the fact that one country poses a severe threat to other countries, so attacking this country might help to maintain peace in the long run. This was the main reason for the Iraq War in 2003, for example. Saddam Hussein, the leader of Iraq, had been suspected of being in possession of illegal weapons of mass destruction; therefore, going to war against Iraq was seen as a necessary action in order to maintain peace.