Read Mark chapter 9, notice the sequence of events.  I can imagine that as Peter, James and John are coming down from the mountain, after Jesus Transfiguration, that they would have been pretty excited.  They had just seen Elijah, Moses and Jesus conversing together.  The occasion was so glorious that Peter didn’t want to leave, rather he wanted to build tents and live there.  Then upon descending the mountain Jesus and the three disciples are faced with the demonic boy, whom the other disciples could not minister to.  However Jesus quickly cast out the demon and set the lad free.  This could be a time of pride.  The disciples start to argue about who is greater.  Of course the three were on the mount of transfiguration, the others could not minister to the young lad.  The three had seen the revelation of Moses and Elijah, it was obvious who was greater.  In this context Jesus says, if you want to be first, be the servant of all.  God is not looking at what you experienced or who you were with, but rather who did you serve.
It is unfortunate that the disciples did not immediately comprehend what Jesus was speaking about.  Right  after Jesus talks about being the servant of all, the disciples were still thinking about who was the greater.  They rebuke those that seem like they were not on Jesus’ team.  The disciple’s mindset was still thinking about things like, “whose team they were on” and “overcoming the competition”.
As humans we want to be able to boast about being on the best team and the experiences that we have seen and participated in.  We fail to do the most important thing, serving others.
Lord Jesus, You came to serve all. Help us to willingly do the same.