1. When Tod and Copper swore they'd always be best friends.
2. When you first heard the words to the main song.
3. When Amos, Copper's master, tells him he's supposed to hunt Tod.
Their whole relationship is a great allegory for war, and the way we're inculcated into being enemies. It's especially deep when you consider famous incidents like that time in WWI when the French and Germans got together to celebrate Christmas, and then went back to fighting each other the next day.