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Sometimes, if you're playing as Luigi and he falls off the track, he will say, "Thank you!" to the Lakitu who rescues him.
In Double Dash!! When you finish the race, the two characters you chose celebrate together. While this leads to the obvious niceness with pairs like Mario and Luigi or Mario and Peach, it becomes really sweet to see Luigi and Waluigi or Mario and Bowser so pleased with each other. Especially during the award ceremony.
In the section right before the cave on Double Dash's Sherbet Land, there are two Shy Guys skating together. It's a small, but sweet sight to behold. Even better, in 8, they'll do tricks.
Meta example: Several fans joined together to play Wii's online one last time together on May 19th, 2014. The next day was the end of the Nintendo Wi-Fi connection, meaning online play was terminated.