Your partner has their own spiritual beliefs and core values, the ideals that make them who they are as a person. When they met you, they sensed -- correctly -- that you also have your own causes and interests that are important to you. If the two of you are together long-term, they might willingly convert to your ideals out of love for you and trust in the relationship.
This is not something that happens right away, of course; after all, it's not like they're a follower, or someone who has no idea what their ideals are and is looking for someone, anyone, to glom onto! No, they have well-formed ideals, and they find that they work well with yours. There may be something in your life, however, that's really important to you -- a religious or spiritual pursuit, perhaps, or a humanitarian cause -- and if the two of you are together long-term, your lover wants to blend their life with yours more and more. Sooner or later, they join you in your pursuit of that important cause, because they want to pursue it together, as a team. This makes you feel wonderful, because it validates all your highest ideals and your deepest concerns. It also brings you closer as a couple, because your lives and interests are merged at such a deep level.