Its a matter of greed.

Here you can see all the greenery in Pakistan whereas Afghanistan is dessert. The dessert parts of Pakistan are inhabited by ethnic minorities, so they dont matter.

Pakistan Is drowning in water and Afghanistan is facing extreme drought. Pakistan is against Afghanistan diverting excess water that leaves Afghanistan, goes into Pakistan, then ends up in the Indian ocean. Pakistan insists on Afghanistan sending all its water to Pakistan so that it may be wasted by ending up in the Ocean rather than Afghanistan using that water to green itself.

-Afghanistan

Afghanistan has tremendous Hydro-Power potential. Pakistan wants Afghanistan to be dependant on ridiculously inflated foreign electricity rates rather than generate its own and to benefit its people.