Though a lot of MBTI description websites discourage an ENFP-ESFJ relationship, many ENFPs seem drawn to the ESFJ. Those MBTI websites often call this a "Benefactor-Beneficiary" relationship. But maybe this is that relationship in a positive light. They worry that once the beneficiary doesn't need anything anymore, they will the leave the benefactor. But in this case, the ENFP will always be in need of someone to tell them how lovely and amazing they are, and to hug them and to tell them it's ok. The ESFJ can provide this phenomenally, and is often just happy seeing the ENFP thrive. I guess it may feel like they fulfilled their duty. I actually don't think that's an unhealthy basis for a relationship, and have often seen that be a basis for a lot of longstanding relationships.