why the high-voltage wire causes the human shocking?

When you grab onto a high-potential power line, your potential is the same the power line. So no potential difference, no current flows thru your body. Then no shock.
If you grab onto a wire of different potential, you will receive shock. Yes different potential makes current flow thru your body. For current higher than 10mA thru your body, your body muscles contracts instantly creating shock feeling. For currents higher than 100mA, your heart fibrillates and you will be dead in 5-10 minutes because blood stops flowing in your body. For currents much higher, your body will be barbecued due to heat.

high-voltage wire means there are high electrical potential energy in the wire,the negative charge has a higher potential energy to transver to the positive one.In addition,since V=IR,Ris constant.Hence,Vhigher,I will higher to.