Being heartless is different from being indifferent. I would argue that a heartless man is a man that cares and he cares because he displays negative emotion rather than no emotion at all.

Being indifferent comes from apathy and apathy comes from constant rejection, heart ache, pain, misery and abject failure. It's far easier for a man to become indifferent when he's been treated with indifference by women throughout his teenage years and his twenties. It's far harder to become indifferent when a man has had some form of success with women in the past.

Indifference comes from knowing that there is no such thing as "the one" but instead there is such a concept of "the one of many". Indifference comes from realizing that women have no power or sovereignty over the direction of your life, that your goals and your ambition are just as important. Indifference comes from realizing that no woman is worth placing on a pedestal and that all women bar family members are disposable entities. If one leaves, another one will come along to take her place. Indifference comes from asking a woman out, getting rejected and taking on the chin without getting upset and wondering if you should get out your Lord of the Rings Boxset and order a pizza to ease the pain.

Indifference: it's a destination of the mind and how someone gets there will differ from man to man. There's no easy to follow formula here.