I think in this case it's a matter of violating community guidelines (in addition to the underlying political reasons Darwin333 mentioned). It's simple. Syria is a member of the United Nations:
http://www.un.int/syria/
Chemical weapons are banned:
http://www.un.org/disarmament/WMD/Chemical/
Syria used chemical weapons:
http://www.washingtonpost.com/world...864662-1196-11e3-85b6-d27422650fd5_story.html
Therefore Syria violated community guidelines. If we do nothing, we send the signal that violating international rules is OK. But then again, there doesn't seem to be much structure as far as enforcing UN regulations via military means goes. And in addition to Britain doing nothing, Obama just put the Syria strike on hold:
http://news.yahoo.com/us-weighing-limited-narrow-action-against-syria-035535999.html
So what are we supposed to do? Just look at high-resolution color images of 400+ dead children killed by chemical weapons in a country that is part of the international community and do nothing, and say gee, let them handle it themselves? I'm not saying that intervention won't leave things a mess (because it will), but I also don't think that standing by and doing nothing while they violate the international rules is acceptable either.