Jack found out he was dead & if I were Jack I'd cry too if I realized that.
And the Reunion was so emotional for me because they went through a lot of pain & loss when they were alive, & they didn't even remember each other in the "Purgatory" world until they did something to regain a memory of that person & the other Oceanic passengers.
Also after their extremely jubilant reunion, they left for a better place...
It made me cry so much.
Reunion after so much pain, suffering, & of course death is such an extremely sad/happy thing!