Why is it called secret love or hidden love? Normally things are only in secret or hidden when it cannot be exposed in the light. When we sin, we sin in darkness and in secret. So, secret love carries many mysterious connotations of a love that cannot come to light. Better is someone who openly rebuke you because this is done in the light and not in the darkness, than a love that has to be hidden.
verse 6 tells us:
faithful are the wounds of a friend; but the kisses of an enemy is deceitful.
A friend will tell you the truth even though it hurts, but an enemy will just say what pleases you because he or she is not sincerely seeking your well-being.
This is another verse that says that kisses or affection may be deceitful but rebuke may be sincere.
This is only my revelation :)