Not quite "all" veterans. I joined after high school because I needed a job and didn't know what else to do. And I had great respect for my dad's military career. Supporting and defending the Constitution had utterly nothing to do with my decision. In boot camp (USN, such as it is) we were taught that the Navy's primary objective was to keep the world's sea lanes open for the free flow of some 53 items which the North American continent did not produce itself in sufficient quantities to maintain the American way of life. No mention of the Constitution there, either. Db is correct, most US wars are intended to generate, or defend, profits.
Was WW2 started by the U.S. to generate profits? How about WW1? Most wars are started by politicians (dictators mostly) who desire to subjugate others. Most of these politicians are not driven to generate or defend profits. I do not disagree that many "war profiteers" hang on the fringes of many conflicts and are solely focused on generating corporate and personal profits, but they are just taking advantage of the situation and political decisions - they are not driving politicians to start wars. I find the current trend that privatizes many formerly military functions in the U.S. to be disturbing. Most of the privatization actually raised costs and under-served our troops in the conflicts in Iraq and Afghanistan.