Pakistan is a U.S. ally. Pakistan shares a border with Afganistan. Pakistan has big economic problems, which have increased the influence of it's large fundamentalist Islamic movement who want Pakistan's government to be Islamic rather than secular. Pakistan would be expected to be caught between the desires of it's ally and occasional patron, the U.S., and it's fundamentalist population. Now, if Pakistan won't help the U.S., effectively attacking Afganistan is almost impossible. If Pakistan will help the U.S., it pisses of the fundamentalists, and maybe, just maybe, it pisses them off enough to overthrow the government.
And maybe that's the desired outcome, because Pakistan has nukes. Not big, unweildy nukes. They have the kind of nukes you can attach to an F-16 (which they also have plenty of) and fly at supersonic speeds to your target.
I wasn't worried before. Now I am.