Knowledge has an end. Knowledge completes. So also does discipleship. For the disciple is aimed at acquiring Knowledge.
Once you cross the water, however nice the boat is, you get off the boat. After twelve years, the disciple completes his studies. The master does a ceremony called Samavartha, where he tells the disciple that he is ending the discipleship and asking him to behave at par with him, and let the Brahman dynamically manifest.
Sakha is a companion in life and death; it never ends. In the path of love there is neither beginning nor end. Sakha only wants the beloved. He doesn't care about the Knowledge or liberation. Love is incomplete because of longing. And so it is infinite, for infinity can never be complete.
Arjuna was a sakha to Krishna and although Krishna was the perfect Master he was a sakha, too. What are you, a shishya (disciple) or a sakha (companion)?