The conversation between the young ladies and the porter touches on the topic of trust, and the importance of keeping secrets. While the young ladies express their fear of confiding in an indiscreet person, the porter reassures them of his trustworthiness, citing his ability to keep pleasant things and sad things separate. This raises the question of what it means to be trustworthy and the importance of keeping promises and confidence. I think the successful secret is the one only you can have.