Extending self.. extending heart. Easy? Have a pet.
Does it not surprise you how calming having a pet is? Soft cat fur and innocent looking eyes which are contrasting to a human glare. When human eyes size you up, cat's stare just convey warmth and acceptance. No need to mask your moods nor pretend to be happy for animals do not demand anything but only kindness.
With cats or other pet animals that you have allow you to extend yourself unconditionally. But I could not say the same thing with most people who would never share anything they have unless returned. Human interaction is always conditional and this stops one from extending self.
The irony of having a pet is finding the true essence of unconditional caring in them.
Maybe that is why they can calm even the man with the toughest heart.