America's interests in Syria are largely tied to a post-Cold War, still-evolving mission of "promoting freedom" around the world, as well as its role as the world's only superpower.
President Assad never publicly antagonized America like Libya's Muammar Qaddafi, nor did his government ever provide overt support for anti-American terrorism, although there is plenty of evidence Syria aided militias and groups in neighboring Iraq fighting against American forces. Additionally, Syria is not a major supplier of oil or other natural resources to America.
Still, since the start of the Arab Spring the Obama administration has cautiously and somewhat inconsistently (see: Bahrain) thrown its support behind the popular movements seeking to topple the region's autocratic regimes, and Syria seems to have evolved into a conflict where it is easy for the U.S. to pick sides.
Secretary of State Hillary Clinton is one of the key players in the "Friends of Syria" group, and any aid package, military or otherwise, will likely have large amounts of American resources involved.
At the beginning of the Syria conflict a year ago, the Obama administration merely called for a peaceful resolution. Now it is firmly demanding that Assad step down.