A thousand years is a very long time to think, in silence and solitude ... How much could Luna have suffered, while her sister reigned Equestria alone? What things could she have felt, thought and remembered?
Exile is always a long process to rethink many issues in life.

Life passes like a sigh to mortals. When you are born, your expiration date is included. These issues, for Discord, were of little consequence, but when he had to face the misery of watching Fluttershy grow old, he realized he was going to lose her forever. Since then, he has prevented death from coming to find his best friend.