Amy Tan, The Valley of Amazement
Full excerpt:
Teddy once told me that it’s natural that we feel alone, and that’s because our hearts are different from others and we don’t even know how. When we’re in love, as if by magic, our different hearts come together perfectly toward the same desire. Eventually, the differences return, and then comes heartache and mending, and, in between, much loneliness and fear. If love remains despite the pain of those differences, it must be guarded as rare…

This remains one of my favourite quotes. It illustrates the requirements of lasting love so perfectly and succinctly.
The quote reminds me of when Jamie observed that there is always a stage in any relationship where there is a power struggle, usually after Year One and during Years Two and Three. The power struggle between two people and their opinions and beliefs–a tug of war to decide who will have the final say over just about anything. This quote says that if we can get through that power struggle stage and still love each other, then that love is rare. I agree wholeheartedly.