When a child acts out of fear, he is acting to please his parents; when a child acts out of love, he is also acting to please his parents. Yet no one would deny that there is something significantly different in these two drives to please. In the first case the child aims to please because of what that will earn him (on Locke's view, esteem and avoidance of disgrace). In the second case the child aims to please so that he can please. His goal is not to earn anything for himself, but merely to benefit the one that he loves. (Locke, who believes that we are only motivated by reward and punishment would not put it quite this way. He would say, instead, that the motivation in the case of love to earn oneself the happiness of seeing the loved one happy. But the point is the same: the focus is primarily on the other, not on oneself.) Motivation out of love belongs to the class that we spoke of earlier as plausibly typifying true virtue: it is like the man who jumps in to save a drowning stranger because he empathizes with that stranger, and not because he wants anything for himself, such as fame, glory, or a clear conscience.
The move to love, in a sense, prepares the child for selfless motivation. Or, more accurately, it trains him in it. Acting out of love is the most common (and, one might argue, the most easy) way to act selflessly. Other types of selfless drives, such as the drive from pure human empathy, are not as strong and, perhaps as a result, not as commonly found as the motivating forces of our actions. A human being, though, who loves deeply and is accustomed to acting out of that love, is in a better position to be motivated by other selfless drives. He is used to acting out of consideration for others rather than just out of consideration for himself. It is not entirely clear whether Locke had this extra perk of love in mind, but it does add to his account of moral development.