Let me explain it in short, and you may not agree with me, but this is the reality, at least a part of it:
As you may know, many Shias of Southern Lebanon have relatives on the other side of the border in Syria which they visit on a regular basis.It's like the same people in 2 countries.
Before Hezbollah entered Syrian conflict, Hezbollah leader, Hassan Nasrallah said he wouldn't enter the Syrian conflict, unless the rogue elements in Syrian opposition start targeting the Shias near the villages on Syrian border to defend them (Actually they threatened to wipe those Shias out for allegedly support of Assad, and then even shelling Hezbollah positions itself). Not only they ignored, but they attacked few Shia-majority villages and violently killed civilians and beheaded many of them only for the crime of being a Shia.
That was the time when Hezbollah stood by its words and entered the Qusair and kicked them out of those areas. Hezbollah entered Syria 2 years after it started and 2 years after rebels and Jihadists came from almost all nearby countries, including Lebanon itself.
Now you may not find it justifiable, but it is what it is.And my friend, before Hezbollah entered the conflict, Syrian war was already an international conflict when rebels came there from 22 Arab countries,Chechen, Turkey and even Europe. So I think decrying Hezbollah's involvement here is actually not fair.