Why did Krishna want Arjuna and Subhadra to get married?
It is said in Mahabharata that everything that happened, had happened for some reason. So, even when the event of marriage of Arjuna and Subhadra took place, it had a strong reason behind it.
So, there were basically two main reasons:
One was Love
When Arjuna was a disciple in the hermitage of Drona, those days he made a bonding with Gada, who belonged to the house of Vrishni. Vrishnis were known to be the part of Yadava’s Clan. Gada and Arjuna made such a great bond, once Gada had described the beauty of his cousin Subhadra who was Krishna’s half sister. After having heard of her beauty Arjuna became curious to know more about her.
While, Vrishni’s were a loyal supporter of the Pandavas, it was obvious for Subhadra had grown up by hearing the great deeds of their braveries and prowess. The skill
of Arjuna with the bow was legendary. Apart from all this, Krishna and Gada had talked about a lot for Arjuna. Resultantly, Subhadra became fond of Arjuna.
Thus both Arjuna and Subhadra were inclined to love each other. Lord Krishna was aware of this and hence he supported them.
Second was Destiny
If Destiny is to believed and Mahabharata reinforces the concept, Lord Krishna was always aware of the fact that Abhimanyu would be a part of Mahabharata War. And Abhimanyu is born to Arjuna and Subhadra.
Hence, to ensure that the grand plan of the universe is put into motion, he ensured the union of Arjuna and Subhadra. Also because of the fact he knew that all the children of Draupadi would be killed in the war. Hence, to ensure that Draupadi had someone to shower her motherly love, it was necessary for Parikshit, Abhimanyu’s grandson and a great grandson of Arjuna, to be born. Parikshit was also required to further the Kuru clan.
These reasons outweighed the promise of Pandavas to Draupadi. Also, Lord Krishna supported Subhadra to win Draupadi over.