It's pretty simple.
Russian intervention has tilted the scales, and Jerusalem is concerned of a regime victory, which would be victory for Iran as well. To prevent that, Israeli officials believe West must intervene in favor of moderate rebels.In fact, the war in Syria has largely served Israel’s interests. The ongoing fighting has worn down the Syrian army to a shadow of its former capabilities. Almost all of President Bashar Assad’s huge store of chemical weapons has been dismantled following an American-Russian arrangement forced on the Syrian dictator in the summer of 2013.
And Hezbollah, Israel’s main adversary in the north, has invested a quarter to a third of its manpower in fighting for the Syrian regime, and is losing dozens of fighters every month in battle. Official spokespeople do not say this publicly – there are things one cannot say when 470,000 people (according to recent accepted figures) are being massacred across the border – but Israel has been quietly wishing success to both sides and would not have been against the bloodletting continuing for a few more years without a clear victor.
Like I said — the U.S. government has been selling our military as mercenaries for foreign interests for years.