Kural - 999

To him who knows not how to smile in kindly mirth,
Darkness in daytime broods o'er all the vast and mighty earth

Explanation
To those who cannot rejoice, the wide world is buried darkness even in (broad) day light.

Tamil Transliteration
Nakalvallar Allaarkku Maayiru Gnaalam
Pakalumpaar Pattandru Irul.