Help me understand Woman in the Dunes.

I just finished reading The Woman in the Dunes.

Ok so as mentioned I just finished this book and I initially found the interesting. The part where he is trapped with the woman confused me quite a lot (mostky his thoughts). I don't know what to think about the main characters except that the man was definitely willing to not give up. The ending was definitely intriguing. But I feel I am missing out on the themes of the book. This is my first dive into literary books and I guess that's why.