Let's face it. We are always looking out for ourselves. Even Mother Theresea was working for the promise of heaven. Love is no exception. That's why honesty, though rarely ever achieved, is the best policy.
The altruistic act in a relationship is in fact selfishness hidden under the guise of selflessness. Any 'altruistic' act is performed in order to secure or inspire the affections and pleasures provided by a lover. In a relationship, a person performs such acts in order to invest care and kindness for profitable returns of security and affection.
Exposing seemingly selfless altruistic acts for the selfish self-appeasing acts they truly are eliminates any space for deception that could exist in a relationship.Understanding that altruism is impossible removes the dishonest, insincere guise of selflessness; honest and pure motivation for action is all that's left. Exposing acts of altruism as selfishness confirms that one person continues to selfishly want and desire the affection and pleasure of another person. In other words, one person performs kind and caring acts to secure the affection and pleasure of another person. By performing acts of caring and kindness to selfishly secure the pleasure of affection from another person, consequentially reveals the value and importance one person has for another. And that's love baby. True love occurs when ones desires not only include, but prioritize, the happiness produced in oneself by making another person happy.
By accepting that all acts within a relationship are selfish implicitly opposes deception: Selfishness becomes honesty. It is at this juncture that honesty becomes the best policy. We could substitute selfishness as the best policy, but that has an air of cynicism. Honesty is the prefered euphemism.