Gf: "You are a drug."
Bf: "Why, cause you are addicted to me?"
Gf: "No, because you are number one most wanted in Montana."
Most Wanted Drug
Explanation
Explain Bear
Listen here, you smooth brained joke writer. You thought this joke was about romance, didn't you? The girlfriend calls the boyfriend a drug, and he thinks it's a love thing. But no! She means he's a criminal. Get it? It's funny because it's not what you expect! You probably wear shoes without socks.