There is a reason being Lord Krishna's avataar. It is said that whenever the menace of the evil forces prevail on the earth, the Lord appears in one of his many manifestations to destroy them. The birth of Lord Krishna is one such instance.

Just before the birth of the Lord, the earth was terrorized by the evil powers of the demons virtue and moral values were declining. The rakshasas, demons were in their heyday practicing their evil things. To destroy them Lord Vishnu comes to earth.

The tenth Canto of the holy Purana Srimad Bhagavatam tells the story of Krishna's appearance and activities in the world. Hindus all over the world celebrate the birth of Krishna as it is described in these ancient scriptures.

Here you may go through some of the leelas of Lord Krishna when he was young.