I imagine Nathan and Cas becoming friends for a reason, Nathan falls into feederism and begins to gain weight but he also visits Cas very often, to ask him for advice on how to have an exercise routine so as not to have extra discomfort as he continues to gain weight, Olí sees to Nathan and sees that with each visit he is fatter, he thinks that Cas is more attracted to Nathan and begins to gain even more weight to "earn" Cas's affection and attention.