h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Ok, so I'm a 14 year old, and I don't know a lot about many wars. I don't really understand why a lot of people have started them and I think what happens in them is really horrible.
Sometimes I think about all the reasons why people have started wars that I actually understand and sometimes I end up thinking, 'Well they had a good reason'.
But thinking about it now I think it must be really difficult to ever have a good reason to say to people 'You should die' or to actually be a person responsible for the death of lots of people, by starting a war.
When you think about all the horrible things that have happened in wars in the past it makes me feel so sad and so angry, so why are countries still having wars?

However, on the other side, I know that a lot of countries in the world could do a lot of damage and they aren't right now. I guess if we all used all the weapons in the world we'd just blow the whole planet up and maybe some of the leaders actually realise this.