The donkey reminds us of the journey Joseph and Mary had to take to Bethlehem.
"He went there to register with Mary, who was pledged to be married to him and was expecting a child."
I cannot imagine what it would have been like to travel from Nazareth to Bethlehem being very pregnant. I've always stayed close to home during the last month of my pregnancies with my bags packed and ready to go to the hospital. Mary didn't have that luxury. Because of the census that had been ordered (Luke 2:1) Joseph had to go to his hometown to register. The timing wasn't convenient for Mary, but it was God's timing. His plan was for the Savior to be born in Bethlehem. He used a Roman ruler to accomplish His plan.
For me, I like to plan things out in advance. But sometimes my planning may not be what God has preordained in His will. Do I submit to His will, or complain that my plans have been interrupted?