After Israel withdrew from Southern Lebanon in 2000, behind the UN-approved "Blue Line," Hezbollah needed to create a fiction that it was still needed to "defend Lebanon." So it claimed that the Shebaa Farms area, which Israel captured in 1967 from Syria, was really Lebanese territory and therefore Israel was occupying Lebanese lands even after the withdrawal.
As far as I can tell, the Lebanese never claimed this land before 2000.
Syrian President Bashar Assad's regime tacitly supported Hezbollah's claim although it never officially conceded that the land was Lebanese.
There are other disputed areas on the border between Lebanon and Syria. The new Syrian regime has signaled that it wants good relations with Lebanon, and while this is not a priority,
that could include resolving the disputes over the border between the two countries.
The international community is very interested in a resolution to this issue as it would ease Syria's being welcomed back as a legitimate state, which everyone seems to want despite the Islamist background of the new regime.
If the border puts the Shebaa Farms on the Syrian side of the border, that takes away one final piece of Hezbollah's pretense of defending Lebanon from Israeli aggression - which is its entire stated reason for existence.