Because you are a citizen of a country. You have to pay some of your money to keep serving your country to you. Basicly lets say, you go to State Schools, your garbage is picked up by state, your state's soldiers&policeman are at work for protecting state e.t.c. Soldier, garbage truck driver and soldier&policeman doesnt work for free, state is paying them. It doesnt matter if you live in a trailer park, or a house. If you are living in a country, you have to pay tax regularly for these services.
Because waayyyy back in the day (during the time texas was controlled by the Mexicans) America wanted the territory, so they fought for it. One famous battle you should know is the Alamo, which was one of the battles for the control of Texas.
Who do you think should pay for all the benefits and social services people want and expect. We have a huge military to protect the country, roads, bridges, services like medicare, medicaid, social security, schools etc. Where do you think that money would come from if not for taxes?
Taxes were meant to be levied on all individuals to create and maintain a social structure that was beneficial to everyone. This system works only in a greed free society which presently does not exist.