I agree with MarineIguana-you are the adult and you are special -you are not best friend-you can say 'no'! (they can get lots of best friends over the years-they only get one mum)
I think it is important to respond to the DC you have and not the one that you would have liked. e.g. you may want to have your DC centre stage in the school play but you have to accept that your DC may want to hide on the back line of the chorus, you may hope for a wonderful tennis player or allround sports person and have an uncoordinated DC who would rather have their head in a book. You may have decided that they will have a glittering career-sailing through school and on to the best university, but they may want to be a landscape gardener or hairdresser. You may be very demonstrative and have a DC who is self contained and reserved.You may be a vegetarian but your DC may not only want to eat meat, but want to be a chef, cooking it everyday. I could go on and on. You have to support the DC that you have and not make them feel that it is second best-it all goes into the unconditional love.