Why impartiality?

There's no such thing as friends and enemies, these are imaginary concepts that actually don't describe anything.

An illustration of this point:

Katayana once saw a man sitting in front of his house, holding a small child in his lap, eating some fish and feeding the child, and throwing the bones on the ground. There was a dog that was trying to get the remaining fish flesh off the bones and the man was kicking the dog away cause it was bothering him.

When Katayana used his super-cognition to see what the karmic precedents of all this were, he realized that the child in the man's lap had in his previous life been that man's worst enemy, in other words that man's enemy in this life had died and been reborn as his child because of the karmic debt.

The fish that they were eating, was the father's father who had been reborn as a fish. And the dog that the man was kicking was his mother who had been reborn as a dog.

So Katayana said about this:
"He's eating his dad, he's kicking his mom, he's cuddling his worst enemy... there's no point what so ever to samsara."

From Kenchen Thrangu Rinpoche (Geshe Lharampa) - Bardo Teachings