Students get bored with the same routine. Students may also think the teacher is a little lazy if he or she doesn't take the time to change the class. A class that is always changing is more dynamic. If a student's group was dull the last time, the next group may be better. I've noticed classes that I constantly change are more closely knit.
Students become more familiar with each other and learn each other's personalities.
And because they share ideas with the whole class and not just with the same old group, their discussions are richer and more interesting. Changing pairings has another benefit-repetition. If a student has to practice a dialogue or language function over and over with the same classmate, that student will quickly lose interest, but if the student changes partners several times, each change will bring a fresh new try at the same dialogue. Try changing the grouping during and after every activity for a week. I'm quite confident your students will respond well.