Because your choice now is either I use my own needle now, or I could share a needle now and get a disease that's going to possibly kill me 10 years from now, or I could use my own needle now and go to jail tomorrow.
因为这时你的选择就是， 要么用现在自己的针头， 要么现在和别人用一个针头 感染上病 过个十年因此而死， 或者我可以现在用自己的针头 明天就去蹲监狱。