Love is painful because it creates the way for bliss. Love is painful because it transforms; love is mutation. Each transformation is going to be painful because the old has to be left for the new.
The old is familiar, secure, safe, the new is absolutely unknown. You will be moving in an uncharted ocean. You cannot use your mind with the new; with the old, the mind is skillful. The mind can function only with the old; with the new, the mind is utterly useless.
Hence, fear arises, and leaving the old, comfortable, safe world, the world of convenience, pain arises. It is the same pain that the child feels when he comes out of the womb of the mother. It is the same pain that the bird feels when he comes out of the egg. It is the same pain that the bird will feel when he will try for the first time to be on the wing.
The fear of the unknown, and the security of the known, the insecurity of the unknown, the unpredictability of the unknown, makes one very much frightened.
And because the transformation is going to be from the self towards a state of no-self, agony is very deep. But you Cannot have ecstasy without going through agony. If the gold wants to be purified, it has to pass through fire.
Love is fire.