Even thouhg your habit of drinking one liter pineapple juice every morning might be one of the reasons why she loves to go down on you, there must be another reason why she ONLY does it with you.
You are special.
You are not like all the others guys she met before. You are one of the few guys who deserve her full sexual service.  You are special enough for her to do something special for you. While she does it she probably thinks about being in a very special relationship with you.