Rashi quotes the Medrash that says that Hashem tells Klal Yisroel: “I have detained you with Me.” Rashi continues: “This is analogous to a king who invited his sons to feast with him for a certain number of days, and when the time came for them to leave, he said: ‘My sons! Please, stay with me just one more day, it is difficult for me to part with you!’”
Sefer Ahavas Shalom explains by saying that most fathers want their sons to get along and love each other. If they get into a fight, it pains their father. If he wants to make peace between them, a good strategy is to arrange a large feast and to use it as forum to discuss their disagreement. The gathering will create feelings of comradery, which the father can use to bring back their love for each other.
So too, Hashem wants His children to be united and to love each other. Therefore, he asks us to remain with Him for one more feast because “kasha ailei p’reidaschem” – it is difficult for Him when we are apart from each other, and He wants us to be united as one.
