The US is generally very reluctant to get involved in the conflict in Syria. There are a number of reasons why some people might want to avoid getting involved. Let us look at some of them.
What happens in Syria does not directly affect the US. Syria cannot directly harm the United States. There is no likelihood that the Assad regime could attack the US. Therefore, the US should not really care about what happens in Syria.
The US cannot really have much influence in Syria. The rebels are not particularly inclined to do what the United States wants. Even if the US gives them money and/or weapons, they still might not turn out to do what the US wants. Therefore, it would be dumb to give them the support.
The US is worried that the weapons that it gives the rebels would end up in the wrong hands. There are definitely some among the rebels who are supporters of Al-Qaeda. The US would not want to give weapons to the rebels and have them end up in the hands of those particular people. It would be hard to prevent this from happening.
These are among the reasons not to get involved.
United States involvement in Syria is a hot issue, with most Americans preferring the country stay out. Three reasons why are:
1) The United States has already been involved in a lot of combat over the past decade or so. Between Afghanistan and Iraq, many lives have been cost and billions of dollars have been spent. Americans are tired of sending troops or planes overseas, and don't want anything to escalate to a point where US lives are being endangered.
2) The United States shouldn't try to police the world and be involved in the affairs of other nations. We have our own problems and should let whatever happen in Syria be their own business. There's always been a belief that the US unnecessarily gets involved in the affairs of other countries when it should just stay out.
3) United States intervention in Syria could worsen already bad relations with Russia. Russia is a friend of Syria, and any opposition to the Syrian government could trigger some kind of response from Russia. Russia has already been a thorn in America's back due to the Eric Snowden issue and their stance on gay marriage, so further disagreements could cause real problems between two powerful states.