Why does Seymour want to marry yuna in ffx?

SPOILER WARNING

Seymour ultimately wants to become the next Sin. In order to do that he must be used as the faith to create the final Aeon. A summoner chooses the final Aeon from one of their guardians. The summoner dies, and the final Aeon becomes the next Sin in ten years. He hopes that by marrying Yuna and becoming close to her she will use him as the final Aeon to defeat sin. This of course would lead to him becoming Sin destroying Spira in order to save it.