Pakistan started harbouring terror and using it as state policy to further its regional aspirations, notably to wield influence in Afghanistan and to destabilise India and fuel unrest in Kashmir.
And now these terror elements have become so ingrained in the ruling establishments of Pakistan, its military and the civilian government that Pakistan doesn’t want to or can't think of getting rid of them.
America's strategic concerns and Pakistan's geographic advantage made it a strategic US ally, first to contain the rise of the USSR in Afghanistan and then to other parts of the Middle-East and Asia or to tame transnational terrorism that began spreading its tentacles from 90s from Islamic countries or America's war against terror post 9/11 the epicentre of which was again Afghanistan.
And Pakistan reaped its benefit while backstabbing America, especially in the post 9/11 period.
The very forces that conspired and attack America's power in September 2001, Al Qaeda and Taliban, got sanctuaries in Pakistan after the US military action uprooted them from Afghanistan. Osama bin Laden, the Al-Qaeda chief, was in fact killed in Pakistan and it is believed that it was due to collusion of the Pakistani establishment that he had been able to hide in Pakistan for years.
A USAID report puts US aid to Pakistan till 2012 at $60 Billion while in last five years alone, since 2011, Pakistan has received $15 Billion in aid from America.
But Laden's location and his killing in Pakistan started changing things. The seed of mistrust was sown when the US didn't inform Pakistan that it had located Laden in Pakistani army's backyard and would eliminate him singlehandedly by breaching the Pakistani airspace.
And it further grew with increasing Haqqani Network attacks in the US. Haqqani Network is an Afghan terror group that operates from Pakistan and has carried our several attacks on the US interests in Afghanistan. The deadly attack on Indian Embassy in Kabul that killed 58 was also carried out by the Haqqani Network.