I was looking at my birth control packet and it said something about not taking weight loss pills, so I was wondering... why don't weight loss pills and birth control mix?

The weight loss pills probably interfere with the absorption of the birth control hormones in the digestive system. If the hormones from the birth control can't get into your blood stream, then they can't do their job. Many antibiotics have this same effect. This is why doctors will tell you to use an additional form of birth control while on antibiotics.

Birth control effects your endocrine system as does prescription weight loss medication.

Second comment is correct... However, it not only blocks the absorption of the hormones into your system, but speeds up your metabolism so that the hormones that are absorbed are processed by your body at an accelerated rate and are less effective!
That is why you should use nutrition and exercise to control your weight loss instead of pills or supplements... Nutrisystem is an excellent resource on proper nutritional needs, content, and portions... and Burn the Fat will educate you upon everyting else that you need to know!