Same reason why western nations were doing business with the Libyans and Iraqis before both were slaughtered by them. Making us ripe for the kill I guess. america is run/controlled by the neocons. Read the neocons manifesto in "the plans for a new american century". Pay special attention to the nations listed for termination.
My friend,
Nations are never really friends.
Geopolitical Interests (of nations) dictate the terms of engagement between them in the long-term.
Example 1:
US had seemingly excellent diplomatic relationship with Iran during the era of Shah. However, Khomeini changed this dynamic. Khomeini not just antagonized the US, several countries in the Middle East felt threatened by his ideological motives; Iraq was one of them. Iraq had good diplomatic relationship with the USSR during the era of Cold War but was an declared enemy of Israel. However, Iraq also had the guts to trade punches with Iran unlike other countries in the Middle East. US saw an opportunity here and provided aid to Iraq during Iran-Iraq war. However, Saddam Hussein's decision to invade Kuwait changed this dynamic.
Example 2:
You must have heard about Sino-Soviet split (1960 - 1989). US and China had traded blows earlier during 1950s on the Korean front. However, US was willing to move ahead and work with China to contain the influence of USSR during the era of Cold War. Sino-Soviet split brought China and US closer to such an extent that US invested heavily in China and transformed it to one of the most progressive nations in the long-term. The world now perceives China as an emerging superpower. USSR is history but Russia and China took a U-Turn and established diplomatic ties.
US is now watching China slipping away from under its influence as it grows stronger and is becoming increasingly dependent to pursue its own geopolitical interests worldwide. In short, China is not a threat and US is thinking to contain it. US is now shaking hands with India and Vietnam....
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Coming towards Pakistan:
American and Pakistani public perceive each other negatively (unfortunately) but Pakistani and American governments have managed to work with each other at times for fulfillment of shared goals in the region. Cooperation in the matters concerning Afghanistan have been defining moment of relationship between US and Pakistan.
Now Afghanistan is once again a test-bed of relationship between US and Pakistan. This time Taliban is the topic of engagement between the two sides. Problem is so-called Quetta Shura and Pakistan's alleged support for Haqqani Network. Additional problem is elements in Pakistan that are sympathetic towards Taliban's cause in Afghanistan.
Another test-bed of relationship between US and Pakistan is the latter's closeness to China. On one side, India is looking forward to contain China's influence. On the other side, Pakistan is granting China an easier access to warm waters.
Looks like Pakistan and US are heading towards collision-course irrespective of what have been stated in a manifesto. If US is looking for a reason to act against Pakistan, we are doing our best to provide it one. We don't need to look at a manifesto to figure out where we are headed.