i interpert this as:
he's really depressed, drinking by himself at night. missing the sun, and someone else. who to him the sun is always with, to me meaning she is always happy. and he doesn't understand this.
maybe he doesnt understand it so much that he "just can't help it, i'm so tangeld up in you." aka he is just attracted to her because of her solid, constant happiness.
so he drinks a lot, and thinks about her a lot too.
i interpert this as: he's really depressed, drinking by himself at night. missing the sun, and someone else. who to him the sun is always with, to me meaning she is always happy. and he doesn't understand this. maybe he doesnt understand it so much that he "just can't help it, i'm so tangeld up in you." aka he is just attracted to her because of her solid, constant happiness. so he drinks a lot, and thinks about her a lot too.
that's just what i take this as.