When I was a kid in a public elementary school, I was taught that holding political office was "a public service," and I believed it. As I grew older, I decided that what I should have been taught was that holding political office was meant to be a public service (and not a career). In an ideal world, only the best and brightest are elected to public office, and these leaders are capable of making the best decisions to benefit their communities, but they also do not seek to be in power forever or to live off others' money for the rest of their lives. But, as we all know, we do not live in an ideal world.
These days I'm inclined to question whether it's even possible for political office to be a public service at all.
In the words of Plato, "Those who seek power are not worthy of that power."
These days (as much as ever), the "dirtiness" of politics is well known. Politicians seek high offices that they can hold for unlimited periods of time or that offer permanent payoff (such as the office of President, which pays over $400,000 a year in office, as well as offering a substantial pension that can result in millions more being made). There's not doubt about it: holding high political office is a lucrative endeavor, especially since many politicians are little more than glorified public speakers (who often don't even write their own speeches) and who are able to raise their own salaries at will.
However, the very fact that we have a government with a system of checks and balances acknowledges that the government is a system in which there is an inherent struggle for power, and as much power as is possible. There are term limits for the presidency because those who instituted the government understood that a person in a position of power will almost undoubtedly try maintain that power for as long as possible until an external force intervenes.
Politics is a hotbed of shady activity, and the opportunity for power and money that public office offers makes politics a tempting banquet for those seeking for selfish purposes, especially those who are manipulative, cunning, or charming enough to ensure their own election. Honest people don't often do well in an arena so rife with corruption, bribery, and underhanded dealings, nor would most honest people be inclined to enter such a den of hungry, carnivorous lions.
Ultimately, the branding of political office as a "public service" is a futile attempt at condoning (and thus validating) the corruption of the political system in the name of some abstract greater good. Our current political atmosphere practically prevents the very possibility of political office being used expressly for the purpose of public service rather than selfish motives.