What does the following poem mean?

If the right people always get along as they did when they first met, they will be so sweet, so warm, so affectionate and so happy. But you and I were supposed to love each other, how could we become estranged today? It's easy to change your heart now, but you say it's easy to change your heart in this world. Just like Tang He, you and I swore an oath of life and death in the Palace of Eternal Life, but we finally said goodbye without any resentment. But how can Tang compare with that year? He has always made a vow to be a pair of birds and connect the branches with Yang Yuhuan.