"Sweet Heart" is a word used for someone who is really very sweet & close to your heart. Any person who is so dear & so close to your heart is called "Sweet Heart". In poetry many times a lover is compared to the Moon. Of course, in the full dark night, Moon looks so beautiful & pleasing to the eyes that it has been compared to one's Sweet Heart, one's lover.
Though God is almighty, all powerful & everyone prays to Him but there is a cult of devotion in which a person falls in Love with the God & there is a burning desire in that person's mind to meet the God. Gopis of Vrindavan, including Radha were of that cult only who fell in Love with the God, Lord Krishna. Meerabai in recent times also considered Lord Krishna like that only. Here below described is the state of mind of a person (Gopi) who has fallen in love with God. This is the state of the highest devotion to God in which a person always yearns to meet the God.
Gopis & Radha's love for Lord Krishna is so strong that in His absence their mind always yearns for Him. In such pangs of separation one Gopi is thinking in her mind about Krishna & saying:
"O Lord, You are my life. I am sweet to You & so is my Lord sweet to me. O Krishna, You are the only one close to my heart. Whom else I need to seek for besides you. Whenever you come into my heart my heart rejoices. You are the Moon of the inner space of my heart. You are always on my mind.
O Krishna! You are Lord of my life. You always remain so in my life. O Lord, don't keep me out of your mind. O Lord, pl. don't forget me.
O Lord, dust of your feet is so pure that if I put it in my eyes, then my vision becomes divine & I can see anything. I want only that your image remains always in my eyes as I do not want to see anyone else besides You.
O Lord, when shall you come to meet me? Hey Krishna, My heart is parching like a desert which is waiting for the drops of rain to come onto it. On the inner space of my heart You never came & burst like a cloud. So many years have gone by but you never came to fulfill my desire to meet you.
O Krishna! My heart is crying for you, pl. do come. My soul is crying for you, pl. do come. O God, there is burning desire in my heart to meet you, pl. do come. Now pangs of separation are unbearable. Now even if I die Krishna but even after death, my desire to meet you shall not die, as all my such desires are indestructible like You only. It shall live on forever till You meet me.
O Lord, I am yours & You are mine, then how come You are not coming to meet me!!! Pl. do come, pl. do come."
Basically God is our own Soul only & lives within our own heart. So when a person turns towards God, then he/she is actually turning towards his own self only. More & more one turns inward & becomes spiritual, he/she actually yearns for God more & more. He/She actually becomes like that "Gopi" only who always yearns to meet the God.