The correct word is "is". Turn it around this way and the answer is more clear: The greatest happiness on earth is to love and to be loved. Happiness is singular, so we use 'is'.

IS...is correct.

Thank you so much Joel!

I believe the quote by Syndey Smith is actually written like this: 'Life is to be fortified by many friendships. To love, and to be loved, is the greatest happiness of existence.' I'm unsure if the grammar is 100% right, but I imagine he wrote it this way on purpose. If 'and to be loved' is only a parenthetical element in this sentence, I don't think the 'is' should be an 'are.' To love (and to be loved) is the greatest happiness of existence. But, I could be totally wrong. Wait until one of the English gurus on italki answers this for you.　 　(*Please give me some advice. Advice is a non-count noun and does not have a plural.)