Arjuna fell in love with Subhadra, the sister of Lord Krishna.
When Arjuna consulted Lord Krishna, he advised Arjuna to abduct Subhadra before marriage.
What are the details of Arjuna and Subhadra marriage?
The story is mentioned in Mahabharata, Adi Parva. Once Bhojas, Vrishnis and Andhakas are celebrating mountain-festival on Raivatak mountain. Arjuna was also present there. There he saw Subhadra and fell in love with her. He consulted Krishna about this.
While wandering there, they saw the handsome daughter of Vasudeva, Bhadra by name, decked with every ornament, in the midst of her maids. As soon as Arjuna beheld her he was possessed by the god of desire. Then, O Bharata, that tiger among men, Krishna, observing Partha contemplate her with absorbed attention, said with a smile, 'How is this? Can the heart of one that rangeth the woods be agitated by the god of desire? This is my sister, O Partha, and the uterine sister of Sarana. Blest be thou, her name is Bhadra and she is the favourite daughter of my father. Tell me if thy heart is fixed upon her, for I shall then speak to my father myself.'
"Arjuna answered, 'She is Vasudeva's daughter and Vasudeva's (Krishna) sister; endued with so much beauty, whom can she not fascinate? If this thy sister, this maid of the Vrishni race, becometh my wife, truly may I win prosperity in everything. Tell me, O Janardana, by what means I may obtain her. To get her I will achieve anything that is achievable by man.'
"Vasudeva answered, 'O bull amongst men, self-choice hath been ordained for the marriage of Kshatriyas. But that is doubtful (in its consequences), O Partha, as we do not know this girl's temper and disposition. In the case of Kshatriyas that are brave, a forcible abduction for purposes of marriage is applauded, as the learned have said. Therefore O Arjuna, carry away this my beautiful sister by force, for who knows what she may do at a self-choice.' Then Krishna and Arjuna, having thus settled as to what should be done sent some speedy messengers unto Yudhishthira at Indraprastha, informing him of everything. The strong-armed Yudhishthira, as soon as he heard it, gave his assent to it.'"
After this, when Subhadra went on Raivatak mountain, Arjuna has kidnapped her.
Meanwhile Subhadra, having paid her homage unto that prince of hills, Raivataka and having worshipped the deities and made the Brahmanas utter benedictions upon her, and having also walked round the hill, was coming towards Dwaravati. The son of Kunti, afflicted with the shafts of the god of desire, suddenly rushed towards that Yadava girl of faultless features and forcibly took her into his car. Having seized that girl of sweet smiles, that tiger among men proceeded in his car of gold towards his own city (Indraprastha).
When Vrishnis have heard about this incident, they became very angry but has been cooled down by Lord Krishna.
When the heroes of the Vrishni race began to speak repeatedly in this strain, Vasudeva uttered these words pregnant with deep import and consistent with true morality. Gudakesa (the conqueror of sleep or he of the curly hair), by what he hath done, hath not insulted our family. He hath without doubt, rather enhanced our respect. Partha knoweth that we of the Satwata race are never mercenary. The son of Pandu also regardeth a self-choice as doubtful in its results. Who also would approve of accepting a bride in gift as if she were an animal? What man again is there on earth that would sell his offspring? I think Arjuna, seeing these faults in all the other methods took the maiden away by force, according to the ordinance. This alliance is very proper. Subhadra is a renowned girl. Partha too possesseth renown. Perhaps, thinking of all this, Arjuna hath taken her away by force. Who is there that would not desire to have Arjuna for a friend, who is born in the race of Bharata and the renowned Santanu, and the son also of the daughter of Kuntibhoja? I do not see, in all the worlds with Indra and the Rudras, the person that can by force vanquish Partha in battle, except the three-eyed god Mahadeva. His car is well-known. Yoked thereunto are those steeds of mine. Partha as a warrior is well-known; and his lightness of hand is well-known. Who shall be equal to him? Even this is my opinion: go ye cheerfully after Dhananjaya and by conciliation stop him and bring him back. If Partha goes to his city after having vanquished us by force, our fame will be gone. There is no disgrace, however, in conciliation.' Hearing, O monarch, those words of Vasudeva, they did as he directed. Stopped by them, Arjuna returned to Dwaraka and was united in marriage with Subhadra. Worshipped by the sons of Vrishni's race, Arjuna, sporting there as he pleased, passed a whole year in Dwaraka.