The whole point of having two terms is to be able to re-elect who we see as the good president while getting rid of the president whose first term was crap. Having only one term for everyone would mean that if you elect the guy who is going to shit all over the country, you will be stuck with him for six years, while if you elect the guy who you want to be in power as long as possible, he will serve as much time as the bad president and you won't be able to re-elect him because his six-year term is over. There is a reason why we can elect a president to multiple terms as well as a reason as to why a term is four years instead of six years.