There are only three basic ways to conduct a political campaign. You can run on past, present or future.
The past means you stand on your record to show the voters they may expect more of the same once you're in office. If you have one they like, you can build a strong presentation from it.
Future means promises. If you are believed by the voters, then you can tell them about what you'll do once in office, and if they like that you're a popular pol who may win.
Present means you focus on the time of the campaign. As a practical matter, that requires you to make your opponent look worse than you do so the voters in a two-party system have to accept you as the lesser of two evils. Negative campaigns of this sort do not concentrate on issues. They're all about personalities.
In the current presidential race in the US, Obama can't run on his record. It is a horrible one, unpopular among the voters to an extreme rarely seen. He has to run away from it instead. Neither can he run on the future. His promises aren't believed anymore even by his supporters. That leaves negative campaigning.
Once a pol goes there, his opponent has to respond. Bill Clinton's rule was that he had to counter a criticism within forty-eight hours, and to do so aggressively, or else it became embedded in the public mind whether or not it was factual. As the news cycle grows shorter, the time frame for response does so as well. What we have today is two pols throwing accusations back and forth in a mudfight. It started with Obama's lack of options. He has to run in the present, not in past or future, and so his options to get himself reelected on any footing but the demonization of Romney are not there. The race has descended into the gutter because the Romney campaign has no choice but to respond to the smears, and doing so requires smears of their own.
Both pols and public ultimately lose from this scenario. Neither candidate will ever fully recover from the dirt, so both are diminished, along with the ability of each to lead once in office. We voters have essentially an issuesless campaign providing us with little in the way of expectations, and so our votes must be cast largely on broad, personal perceptions manufactured by professional political operatives. This campaign is damaging to everyone, but it's what we get this time, and the reason is that Obama has no chance of reelection without turning it into a mudwrestling contest.