The US, much like any other nation out there, is very much concerned about its own national interests, with one of those interests being the safety of our people both at home and abroad. If this plane went missing and no US citizens were reported to be on there, I'm sure our involvement wouldn't be the same.
On the other hand, there are recent reports stating that this could have been a potential terrorist plot, in which case I believe the US also has an interest in thwarting terrorism abroad, again for its own interests.
We were helping Malaysia because the plane that went missing was an American made plane (Boeing 777) and the NTSB (National Transportation and Safety Board) has helped investigate several accidents abroad in the past. This isn't a rare occurrence for the United States to get involved in an international investigation.