Dr. Manette and Wesley are perfect examples of this. He is loyal to Lucie under all circumstances. This is shown by his daily devotion to her, as he forgets about everything else just to make her happy. Similarly, Wesley is devoted to Buttercup in a very similar fashion. He sacrifices everything else just for her. He gives up his own personal rights and desires, for the mere chance of having her love back. Secondly, both Manette and Wesley show loyalty even when they think they have lost what they are being loyal to. Wesley is truly convinced that Buttercup loves Prince Humperdink, while he still remains loyal to her. Likewise, Manette remains loyal to Lucie although he barely even knows if she exists. They both lost what they loved and are willing to do anything to get it back.
Finally, Lucie and Buttercup also display loyalty. While they both think they have lost their loved one, they are loyal to them once they realize that they have them back. Lucie thought that Wesley was long gone and dead, but the second she realized that he was the man in black and that he was alive, she remained loyal to him. In the same fashion, Lucie did not know that Manette was alive, but when she met him she helped to bring him back to life; she remained loyal to him.
In all four of these characters, loyalty is displayed and its result is the preservation of life. These characters were loyal to each other, which eventually led to them saving each other's lives. Just as sacrifice caused death, their loyalty caused life.