Why Won't You Die is a gear perk that improves your health regeneration. It works at all times, so you recover from fights faster without healing by hand.
Your health refills faster after taking damage. You spend less time waiting between fights.
Wear the Vaas Tank in the chest slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
A good all-round pick, especially if you take a lot of chip damage.