At one point in life, everyone changes; that person of before changes. The person becomes either cold and heartless, or sensitive and naive. Heartbeats slow down due to indifference in daily things or even surprises, or speed up due to any type of reaction throughout their life. Yet even the cold and heartless people at times feel pain, I believe. I believe there is a trigger, a switch in each and one of our beings, a switch that has it's hot and cold, a switch that can trigger a specific reaction in deep moments. Even the heartless feel heartbroken sometimes, alone, and sad. Not only due the sensitive and naive people get heartbroken or hurt, no, the heartless due to. 
Of human kind there is only one, and though many may choose to be "classified" as different from other human beings, they are all similar. All with a trigger, all with a switch that turns on in the moment of a feeling and off, in the moment of numbing oneself to reality.