A human wishing to marry God in deity form?! What does this mean? This mood is terms as Madhura Bhakthi bhava (the mood of sweet devotion) in which the seeker (soul) strives to unite with God through an unconditioned loving exchange.
How did she establish such a loving mood with the deity form of God? Her father (a great devotee himself, Vishnuchitthan Alwar) narrated stories of God (especially Sri Krishna) during Andal’s childhood. This inculcated deep love for God in her heart. Her love was so sweet, so pure that even God’s supremacy was absorbed into it! This is the union, the wedding of the (soul) soul with paramaatma, God! Proving the truth in her love, God accepted her and made her available to all of us in the form of a Deity, so we can also worship her for showing us a beautiful path to eternal bliss through love!