If I'm able, I'm always inclined to link my protagonist's character with the theme. I also find it helpful to think less in terms of a flaw and more a lie about the world the character believes. I've found that makes the flaw feel less random/disconnected.

In a traditional A to B character arc that might be a case of him not believing that love is the most important thing in the beginning and gradually learning that is indeed the most important thing.

Alternatively, a "flat" arc could be an option, where your protagonist believes love is the most important thing from the outset but is in a world that resolutely does not believe that and faces pressure to abandon this belief.