Xanax in the benzo class of drugs is addictive because it depresses the central nervous system quickly and wears off quickly due to its short half life. Each time it wears off, your nervous system attempts to get back to normal by speeding up. This speeding up process causes rough side effects which can only be relieved by taking another dose.
No, it doesn't have opiates in it. As said, it's a benzo class of drug, which binds to specific sites on the GABA-A gamma-amino-butyric acid receptor, essentially taking over from your normal system by binding to these post synaptic receptors, which causes the pre-synaptic neurons to go dormant, or depopulate from lack of use.
When you try stop taking those pills, those neurons aren't there to instantly resume normal function again, so the post synaptic receptors don't receive any or very much of your bodies own neurotransmitter- namely gamma-amino-butyric acid which is normally produced and delivered by the pre-synaptic neuron. Therefore you get bad reactions.
(the tiny space between the pre-synaptic neuron and post synaptic neuron is called the synaptic cleft, and is where neurotransmitters are deposited to be picked up by receptors on the post synaptic cleft.)
That's the real scientific explanation, a good one given by 'ovitall'. As for me, I always prefer to go the simpler route (you can picture in your mind) to scare you and others from starting the use of these benzos. I was on them and suffered the results. When you depress the nervous system with chemicals, it will spring back when the chemicals start to dissipate making you feel some awful side effects which can only be relieved by taking more. That begins a dangerous cycle over and over.