You have your own spiritual beliefs and core values, the ideals that make you who you are as a person. If you think back to when you two first met, you'll remember that you sensed -- correctly -- that they also had their own causes and interests that are important to them. Over time, if the two of you stay together long-term, you might willingly convert to their ideals out of love for them and trust in the relationship.
This is not something that happened right away, of course; after all, it's not like you're a follower, or someone who has no idea what their ideals are and wants someone, anyone, to glom onto! No, you have well-formed ideals, and you've found that they work well with your lover's. There may be something in your lover's life, however, that's really important to them -- a religious or spiritual pursuit, perhaps, or a humanitarian cause -- and if the two of you want to keep this relationship strong, you'll want to blend your lives more and more. If you haven't yet, join them in their pursuit of that important cause, because you should pursue it together, as a team. This can make them feel wonderful, because it validates all their highest ideals and their deepest concerns. You grow closer as a couple, because your lives and interests are merged at such a deep level.