For both parties to the marriage, it is a happy event, so it is used first.

Then, because in the folk tradition, it is second to paste happy characters for good looks.

In addition, the word "Xi" also means double happiness, not only because the two parties get married, but also because they will have children besides ending their extramarital affairs, and both parties will celebrate together.

As for the reason, it is related to Wang Anshi in the Song Dynasty (folklore)

Wang Anshi went to Beijing to catch the exam and passed by somewhere. As it was getting late, he decided to stay and have a rest. I have nothing to do after supper. It's nice to see a lamp hanging outside the yard of a big family. Wang Anshi looked closely. There is a pair of semi-couplets on the lamp, which says "Go, light and dismount". Obviously, it is waiting for someone to make a bottom line. Wang Anshi saw that the couple agreed, but failed to come up with the bottom line and left angrily.

Later, after Wang Anshi handed in the first paper in the examination room, the examiner intended to test him, pointing to the flying tiger flag in front of the hall and saying, "Flying tiger flag, flying tiger flag, hidden tiger flag." Wang Anshi immediately thought of half a pair of lanterns outside the stable. He replied without thinking: "The lanterns run, the lights go, and the lights stop." This surprised the examiner.

After the exam, Wang Anshi came here again on his way home and was recognized by the housekeeper of a big family. Please make a pair at home. Wang Anshi changed flowers and casually wrote: "Flying Tiger Flag, Flag Tiger Flying, Flag Tiger Hiding."

This big family is very happy, because this is a married couple, so they married their young lady to him. On the wedding day, Wang Anshi wrote a big word "Xi" on red paper, which was slightly drunk and made people stick it on the door, casually chanting: "A couplet makes a double happiness song, and a lantern flies with silk knots."

Since then, the word "Xi" has spread, and since then, the word "Xi" and the double happiness in marriage have become popular among China people.