Any time you let someone do anything for you, you give them power over you. The ammount of power you give them is directly related to the importance of the task and your willingness (and/or ability) to do the task yourself.
When you let your friend get you a soda, you are giving your friend power over your soda fullfillment. This is a very minor aspect of your life, and you'd likely be fully willing and able to do it yourself if your friend decided to pull some prank, so there is very little power involved -- but it's still there.
Living in this society, we have garbagemen (technically could be woman, but the name stuck over the years). We pile up all our trash into these containers they approve of, place them in a particular fasion at a particular place on a particular day of the week, and they take our trash to the government-approved disposal location. While the government does allow us to take the trash there ourselves, we are charged for the privilage, and it's not usually anywhere convienant. Hence the garbagemen have power over our trash disposal. They can decide that the containers must look a particular way, and we are forced to comply. They can declare my placement of the trast container to be unworthy, and my trash won't get taken away (happened once). They have the ability to make me perform damn near any sequence of actions relating to my handling of the trash before they take it away.
The more people rely on you to do things, the more power you have.