For a nice detailed answer turn to the BBC:
http://news.bbc.co.uk/1/hi/world/middle_east/4308823.stm
It is quite complicated as this will explain -
"Syrian governments have always regarded Lebanon as part of "Greater Syria". They argue that Lebanon was carved off from the Ottoman province of Syria by France, the colonial power in Syria and Lebanon after World War I. According to this reasoning, the Syrian influence and presence in Lebanon is perfectly natural."
"Syria's influence in Lebanon is its strongest card in its continuing conflict with Israel. Damascus has used its Lebanese ally, Hezbollah, to keep up military pressure on Israel's northern border.
Damascus says it will only stop doing so - and withdraw from Lebanon - in the context of an Israeli-Syrian peace agreement, and an Israeli withdrawal from the occupied Golan Heights.
Analysts say that if Syria is pressured out of Lebanon, without an agreement with Israel, it is in danger of losing its Lebanese card without gaining anything in return. "