John 15: 1-8
In this short passage of just eight verses, Jesus uses the word "remain" eight times. It is not hard then to see what his emphasis is.
It is worth noting that in the John's Gospel, this passage forms a part of Jesus' last words to the disciples during the Last Supper. More significantly, by this time in the evening, Jesus has predicted that one of the disciples would betray him, and that Peter would deny him (Chapter 13).
Hearing these words then, the disciples probably were not fully aware of how true and how essential to remain with Jesus was.
We, the readers, and disciples of Jesus today have the benefit of knowing what happened to Judas, Peter and the other disciples when they chose not to remain with Jesus. We also know how they changed after they had witnessed Jesus' resurrection and received the gift of the Holy Spirit.