Humanity

I've been watching Vampire Diaries lately. I am fascinated, in an almost obsessive way with the concept of a humanity switch that you can turn on and off. When on, it enables you to feel everything (the pain and the joy, the love and the hate). When off, you go through the motions without any remorse, compassion, attachment. You seek pleasure but feel nothing deep. You can be free of whatever haunts you. It's not exactly eternal sunshine of the spotless mind, because you remember, you just don't care.
In your darkest moments, when you feel all the worst things life can bring (fear, pain, sadness, disappointment, loss, unrequited love) and you want to shut it all off - what prevents you? Is it the love of another? What can trigger you back to feeling it's all worth it? What makes you struggle to hold on to hope and kindness and love even when you feel incapable of forgiving? When you feel unable to move on? What prompts you to put one foot in front of the other when the pain overwhelms you? What gives you faith in humanity again?
The ultimate, and most terrifying question is what happens when you allow your humanity to stay on and then something else happens that shatters your reason for keeping it on?