Under the current system, incumbent presidents have to begin thinking about re-election as soon as they are elected. After serving their first two years, they have to begin fundraising and, essentially, preparing their run for re-election. This work intensifies in the 4th year. Thus, presidents cannot devote their entire time and energy to their administration, but must worry about media coverage, popular opinion, opposition from their own party, and opposition from the opposing party.
If a president were elected for a single six-year term, re-election would not be a distraction. If the president's administration was successful and popular, the vice-president could run for office on the platform of continuing them.
Senators serve 6 years. Presidents should too.