You describe a very common "bias" in the health care field. There are several reasons behind these feelings, including:
1) our own family histories (is someone in our family chemically dependent?)
2) our culture (we have a drug "war" going on)
3) the biases of our instructors and colleagues
4) lack of formal training/education in pain and pain management
5) lack of an objective pain monitor (i.e., there is no "dynamap" or "thermometer" for pain readings. We have to rely on what the patient tells us, which can be a scary thing, especially if we "believe" patients will become addicted to intravenous medications.
Margo McCaffery, RN has an excellent book called: "PAIN, A Clinical Manual for Nursing Practice" (McAffery and Beebe). You can order the book through any book store or from Amazon