A strong Hisbollah is Syria's only defense for it's long right flank. Israel refuses to allow a strong Lebanese Army, and refuses to pursue peace with Syria. Syria, being far weaker than Israel does what it can to defend itself. Israel is THE power in the region, it's actions and policies determine what everyone else does.
Rohani: Nuclear deal achieves Iran's goals (AP)
from the article: 'Syria helped Hezbollah amass some 40,000 missiles'