The Part 2 of the concluding commentary on Chapter 1 gives insight into how Arjuna's dejection can be called a Yoga, the significance of the number 18 in Bhagavadgita, Mahabharata and the Puranas.
Arjuna gives up fighting even after submitting to Lord Krishna, so the Lord cheekily says to Arjuna that he is talking like a great scholar while grieving for others when he is not supposed to.
Arjuna realises he is overcome with an uncharacteristic despondency so powerful that he is unable to choose the righteous path that is best for him. So he seeks refuge in Krishna as his Guru to guide him.