Because the author came to Xiaoshitang with friends under the background of being relegated, because he saw the beautiful scenery of Xiaoshitang, he could not resist his happiness. However, this happiness is temporary, because the small stone pond is deep and cold after all, and there is no one but the author and friends. This environment stimulates the author's depression and sadness, so it is not contradictory.
It was after he was demoted that the author sent his feelings to the mountains and rivers, so as to dispel his inner resentment and forget his troubles. But once this temporary joy is triggered by the bleak environment, the sad mood will be revealed. It can be said that "happiness" twice is to set off the "sadness" below.
Another example is Li Yu's "Looking at Jiangnan": "How much hate? In my dream last night. It is also like the old garden, the car is like running water in Ma Rulong, and the flowers and the moon are spring breeze. " In poetry, the poet began to express "hate", that is, the pain of national subjugation, and then described the prosperity of the "old garden": the traffic was busy and the flowers were spring breeze. The poet's "hate" is all in this "prosperous grand occasion", and the connection point between love and scenery is the sentence "Last night was in my dream" here. The more beautiful the "old garden" is depicted, the more the poet can express the pain of being imprisoned after waking up. From the relationship between scenery and emotion, this is the above-mentioned "writing sadness with joy"; In terms of expression, it is called foil: the pain of national subjugation contrasts with prosperity.