When you think about it, the belief that happiness comes from controlling external conditions is undoubtedly the fundamental cause of all wars.
[The] innate world view that happiness and suffering come from external sources leads us to believe that if we could only make others and the world be what we wanted them to be, then we would be happy. Thus, we endeavor to rearrange the world and the people in it, gathering towards us those we consider happiness-producing and struggling to be free from those we think cause pain. Although we have tried to do this, no one has succeeded in making the external environment exactly what he or she wants it to be. Even in those occasional situations in which we are able to arrange external people and things to be what we want, they don't remain that way for long. Or, they aren't as good as we thought they would be and we are left feeling disappointed and disillusioned. In effect, the supposed path to happiness through external things and people is doomed from the start because no matter how powerful, wealthy, popular, or respected someone is, he or she is unable to control all external conditions.
The excerpt above is from a longer piece by Thubten Chodron you can find right here.