The woman sings her love song to her husband. As she does so, she remembers that she cannot always express her love as much as she would like. It is not always possible to be as intimate, as confidential with him as she would like at that moment.
In those days, no one had a problem with you playing with your brother and then openly giving him a kiss. But you would not do that with your husband in public. That was indecent in the eyes of the people. You would be criticized. You were not allowed to show your feelings for your husband in public. In fact, kissing your husband in public was almost impossible. If you were outside, you had to keep your distance. You can also see this in Song of Solomon 1. There the wife wants to know where her husband is in the afternoon with his sheep. She wants to make sure that she can be with him in the field, without his friends being there as well. We read in Song of Solomon 1:7, Tell me, you whom my soul loves, where you pasture your flock, where you make it lie down at noon; for why should I be like one who veils herself beside the flocks of your companions?
If her husband had been her brother, it would have been wonderful. Then she could have given him a kiss in public without further ado. It is precisely that deep friendship and profound love for her husband that gives her the desire for that intimate relationship with him.
1 Oh that you were like a brother to me who nursed at my mother’s breasts! If I found you outside, I would kiss you, and none would despise me.