I think Dafoe just loved the light because he was crazy. Pattinson wanted his approval and Dafoe would never let him tend to the light no matter what. So he went mad and did anything he could to see it, convincing himself it did have some power over Dafoe. It's also worth keeping in mind that isolation, no water, booze, repetitive shitty meals, no smells but salt air and farts, hard labor, verbal abuse, and no sounds but waves and seagulls drove them both mad. I think both really just want love. Dafoe is divorced if I remember correctly and Pattinson ran away from a man, even "taking his name," which I took to mean he couldn't understand or confront his feelings about him. They both decide to get love from the light since Neptune knows they aren't gonna get it from each other.