The birth of Hizbollah changed the facts on the ground (eventually). Hizbollah ended the long Israeli occupation of Lebanon. If not for Hizbollah there would be Israeli settlements in south Lebanon today. In order to encourage Israel to respect the border, a strong Hizbollah is necessary. The Lebanese army will not serve this purpose as they are subject to the whims of politics. The Lebanese army did not lift a finger in defense of their nation during the 2006 war.
U.S. and Russia discuss flight safety in Syria on conference call (Reuters)
from the article: 'Syria helped Hezbollah amass some 40,000 missiles'