Pakistan has always been willing to do what the US says, so long as the price is right. For example, during the Cold War Pakistan helped the US fund militants in Afghanistan. Pakistan's willingness to cooperate is something that comes in handy, where as India is a more non-aligned country that prefers Russia to the US.
Also, there comes the case of Afghanistan. Pakistan is used to get NATO supplies to Afghanistan, and if the US gets on Pakistan's bad side, those supply lines will be cut (as they have been before), and the Northern Alliance will start to starve. Not to mention Pakistan has a fair amount of influence over the Taliban. If the US gets us angry, we can tell the Taliban to stop talking and start shooting, and we will gladly supply them. If the US gets in our good books, we can tell them to perhaps be more lenient and we can threaten to cut supplies. The fact that we support the Taliban is a very useful tool, a bargaining chip with the US if you will.