Naruto shippuuden volume 47 ch.445

So Nagato goes on with his story and explains that the two Konaha ninja's then suddenly died which he just recently realozed that he had done. He then wandered around his broken land in search of food and found a dog that he named Tiny. He knocked on every door and was turned down by everyone because they didnt even have food for themselves. So he laid down in the street and waited there hoping he wouldnt die from his hunger..

Then a girl spoke to him that was about his own age and offered him bread. He wasnt sure at first but then he thankfully took it and shared with his starving dog. She then had him follew her to where she was living, where he learned her name and met another kid his age. Her name was announced as Konan the same lady that is Nagato's pawn now, the other kids name was Yahiko the Pain that Naruto just beat to get to Nagato.

Together they stole fish from there area and struggled together to survive and became good friends and they kept the dog. Then the three were walking together and were telling Nagato he should have dreams so he can grow up and have something to look forward too. Nagato expressed his dream of being on top of the world so he could end suffering and possibly stop war.
Nagato held on to this dream as his own because he felt the same way.

Then the three were attacked and the dog got injured they recognized one of the people fighting as Hanzo their amegakure. They also realized the dog stopped breathing and they had to escape. Nagato then told Naruto that the people fighting their amegekure was the pervy sage, the old lady hokage and orchimaru the three warriors of Konoha. Then with rage Yahiko stated that if he had a dream it would be to become the god of the world, which Nagato definately took hold of as his dream.