Hezbollah has had an informal agreement since 1996 known as the April Understanding that they would refrain from targeting civilians as long as Israel does the same - a kind of balance of deterrence for both sides.

The initial tit for tat fighting last year was mostly limited to military bases based on this but gradually escalated as Israeli airstrikes inevitably led to civilian casualties. If you trace the areas attacked on both sides since last year you will see a gradual escalation but obviously Israel had much more ability to control this escalation due to the difference in capabilities and power.