Because love is an act of faith.

Nothing that is worth doing can be achieved in our lifetime,
therefore we must be saved by hope.
Nothing which is true or beautiful or good makes complete sense in any immediate context of history; therefore we must be saved by faith.
Nothing we do, however virtuous, can be accomplished alone;
therefore we must be saved by love.

- Reinhold Niebuhr

And now these three remain: faith, hope and love. But the greatest of these is love."
-- 1 Corinthians 13:13