The song is from season 8 episode 23 of MLP:FiM.
Main song: https://youtu.be/RqtmcBxJyWw | ♫ Lyrics for the song:
( You can support the show here: https://itunes.apple.com/us/tv-season/my-little-pony-friendship-is-magic-vol-15/id1357399950 )
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[Autumn Blaze]

'Cause rainbows won't light up the sky
Unless you let it rain
And candles just won't glow until they're burned
No, you can't give up your laughter
'Cause you're scared of a little pain
It's a lesson that my village never learned

Oh, yes, 'cause rainbows won't light up the sky
Unless you let it...

Raaaaaaaain!

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"My Little Pony" is owned by Hasbro,
"MLP: Friendship is Magic" is created by Lauren Faust.