One hopes, of course, that a relationship grows and becomes a deep and wonderful marriage and friendship that lasts forever. But that's not always the case.

This quote is talking about relationships in general, and how people often hope that the relationships they are in will grow into long-term, strong marriages and friendships. However, this is not always the case. Sometimes relationships can break down and lead to divorce, or the couple may remain together but not have the deep and wonderful marriage we all dream of.