The Only Sin

When did not caring become a herald of human evolution?
More importantly, why is it okay for people to be indifferent?
I do not mean in the basic sense where not bothering yourself with others' opinions, world standards and imposed expectations is wrong. It's needed, healthy even. It's essential.
 But what never ceases to render me incredulous with disbelief is the blatant, unabashed insensitivity of one human being to the emotions and suffering of another human being.
Is it too much to expect of fellow individuals to not shrug and walk away in the face of another's pain, with the toss of head dismissing all responsibility in a thought bubble, screaming 'Not my problem. Not mine to care'?
When did it become tolerable to suffer the indignity of apathy? Why do we not stop to think about how it is the only real sin which keeps us from ever fulfilling the potential we have of being downright decent human beings?
We complain about a ruthless world, literate but uneducated masses' habits, wrinkle our noses in disgust at the sight of an injured animal, littered waste and naked children playing in poverty under the scorching sun.
How many of us have actually ever spared a thought beyond the instantaneous reaction to consider how we could change their situation?
As an adult witnessing the world change around me in the span of every breath, I feel ashamed. I feel disappointed by how people refuse to be affected enough, on the defence of being indifferent to suit their purposes, to ever step out of their way to sit in silence with a friend, to reach out to a stranger in times of their apparent distress, to love a stray puppy, to throw trash in the dustbin.
How do we expect to change the world when we can't change lives?
How do you intend to be a better person if you're too busy living in the false comfort of an impenetrable bubble of this repulsive emotion?