Any instructor will tell you that the martial arts begin and end with courtesy. Why is this?
One reason for rules of etiquette is that aikido has no rules! Its techniques are based on fundamental principles. But in action, the principles are combined without pattern, premeditation, or rules. This requires great concentration! Behavior not directly related to waza (like how we enter the mat, or ask the instructor for help) is standardized so that our fellow students are not distracted from what is really important in the training.
A greater reason is that courtesy is the outward form of respect, and we hope to develop respect for our partners in practice. When we respect someone, we pay attention to his needs and wants, so that we can harmonize with them. Have you heard the phrase "to know her is to love her"? By paying attention and knowing our partner, we learn compassion — and, if we are lucky, the universal love that is at the heart of aikido. We can't see or enforce respect and love, but we can and do enforce courtesy in the dojo.