山盟海誓亦会段
[shān méng hăi shì yì huì duàn]
Mountains can pledge, seas make an oath, but they too could come to an end. It suggests the fragility and changeability of relationships; it is possible that the solemn vows will ultimately be broken even if they were once like solemn vows on mountain summits and beside seas, and all the deep affection may come to naught eventually.