Tomorrow is my wedding anniversary. It will be 41 years. I've bought a card for my wife without a message because this year I'd like to include a quotation from literature or poetry about the nature of love. However, I'm stuck! I'm looking for something akin to "The quality of mercy is not strained" that reflects the depth and qualities of long term true love. I'm only looking for about 4 lines max and I've tried google and an anthology of favourite poems, but haven't found anything that hits the right note. Could anybody with a better knowledge of English literature than mine help please? All suggestions gratefully received.