Bhagavath Yamunachaarya in his Gitaartha Sangraha says:
Arjuna saw his relatives like Bheeshma and his preceptors like Drona who have assembled in his enemy side eager to fight with him in the kuru-kshetra battlefield. They where once very kind and affectionate to him. But now they are in his enemy side. On seeing them like this, Arjuna got his mind agitated. His love, attachment and mercy overflowed regarding them who are now actually not fit for that. He mistook his kshatriya-dharma, which is to fight in the war as adharma.
He then became frightened to kill the enemies in the war. He lost courage to fight and with trembling body, dropped his bow and arrows and sat down tired in his chariot with terrible confusion.
He then surrendered to Shree Krishna Bhagavan who was his driver of his chariot. He requested Shree Krishna Bhagavan to kindly direct him.
Shree Krishna Bhagavan mercifully accepted Arjuna as his student and wished to clear Arjuna's doubts & confusion and direct him towards all goodness. Shree Krishna Bhagavan with his divine gentle smile in his divine lips, in order to teach him the essence of Veda - the tatva-traya, hita and purushaartha - the secret of all saastra in an extraordinary fashion wanted to start His sermon - The Bhagavath Gita.
This is the essence of the first chapter of Bhagavath Gita.