Feenix you have an excellent answer. The source, you are partially correct. Lobbyists represent companies that provide vital products and services to the country, such as oil.
Another major reason is that we fight battles to stop aggressors from invading other countries that are allies. We fight to protect human rights. We fight to protect our own interests.
The debate about the U.S. being the World's Police Force has been going on since I was in grade school (black and white TV, dial phones, party lines and nickle soft drinks).
I just think, and some of you will say this is sappy, it is our nature as a country to help other countries that do not have our resources, freedoms and capabilities.
I will not disagree with anyone that we are over-extending ourselves and that our foreign policy needs a much closer look.