The advent of Dhaumya - Visiting Mahabharata 38

It was midnight. Pandavas reach river Ganga and prepare to take a bath. A gandharva was there with his wives and resents the intrusion. An argument ensues and Arjuna claims that the sea, the slopes of the mountains and waters of rivers are common to all!  Predictably it escalates to an armed conflict and the gandharva shoots arrows at Arjuna. Arjuna uses the Agneya astra and sets fire to the chariot and pulls him out. Wives of gandharva fall at the feet of Yudhisthira and beg for mercy. Released, he wants to be friends with them.

The gandharva seeks information on the agneya astra and gifts Arjuna the power to see the happenings in all the three worlds and horses that knew no fatigue. Learns that they are Pandavas and speaks of the necessity of a kulaguru for future kings and suggests Dhaumya. Arjuna thanks the gandharva and asks him to take care of the horses for them till their circumstances have changed for the better and takes an affectionate farewell.

They seek Dhaumya, who impressed with their humility and behaviour agrees to be their guru. Feeling that dark clouds were lifting, Pandavas hurried towards Panchala where, according to Vyasa, great good fortune awaited them.

There must be something about Arjuna and the Pandavas. A defeated Drupada admires Arjuna for his skills and wants him for a son-in-law! Now it is this defeated gandharva who befriends him. (The Gandharvas generally had their dwelling in the sky or atmosphere, and one of their offices was to prepare the heavenly soma juice for the gods. They had a great partiality for women, and had a mystic power over them. The Atharvaveda speaks of "the 6333 Gandharvas." Chitraratha was chief of the Gandharvas; and the Apsarases were their wives or mistresses. The "cities of the Gandharvas" are often referred to as being very splendid.)
http://www.mythfolklore.net/india/encyclopedia/gandharva.htm