A deal can go two ways

Bill and Dipper have been best friends since he freed Bill from his stone cage.

Bill and Dipper made a deal that Dipper will teach him about humans (emotions and needs) while Bill will teach him demon magic.

While Dipper knows human emotions he tries to keep his emotions a secret. While Bill, who doesn't have any knowledge on human emotions tells Mabel he's emotions that he is feeling and keeps the emotions away from Dipper away from Dipper (Despite him being clingy to him).

(Dipper is 15/16 in this chapter and he will be 15/16 unless I say that he is 17 or 23)

(Bill is 17/18 in this chapter and he will 17/18 unless I say that he is 19 or 25)