When Reshiram/Zekrom saved the other from the out of control Dragon Force energy during thier fight. It was nice to see that despite being on opposite sides for so long, they still care for each other.
When Ash is chosen by Zekrom, it's his honest desire to free Victini that awakened the legendary dragon.
Zekrom: What is your wish? Your ideal?
Ash: Uh! (thinks about how sad Victini has been) Ideal? I really wanna help Victini. I've gotta help! Victini lived here all lonely for a thousand years. Lonely, must have been so lonely. I promised I'd take Victini to the ocean.