Imagine this scene: a layman sits in front of his house, eating a fish from the pond behind the house, holding his son in his lap. The dog is eating the fishbones and the man kicks the dog. Not an extraordinary scene one would think, but ven. Shariputra commented:
"He eats his father's flesh and kicks his mother away,
The enemy he killed he dandles on his lap,
The wife is gnawing at her husband's bones,
Samsara can be such a farce."
What had happened?. The man's father died and was reborn as a fish in the pool, the layman caught his father, the fish, killed it, and was now eating it. . The layman's mother was very attached to the house so she was reborn as the man's dog. The man's enemy had been killed for raping the man's wife; and because the enemy was so attached to her, he was reborn as her son. While he ate his father's meat, the dog - his mother - ate the fish bones, and so was beaten by her son. His own little son, his enemy, was sitting on his knee.
Samsara certainly is complicated... and we further complicate our lives with meaningless activities which collects only negative karma...
When we think about it, if we truly believed in karma and reincarnation, we will never be mean to anyone ever again... because the people around us could have been our parents in a previous life. In fact, after millions of rebirths, we definitely have crossed paths with the very people that are beside us right now. The kindness of one's mother cannot be erased even if it was a mother from another life... she would have loved us just like how our mother in this life loves us.