Nuking another country is no joke. Just because we "suspect" that the other side might nuke us, we can't justify emptying our arsenal to create a wasteland of the other country.
Intelligence about a probable nuclear attack on our country can never be fool-proof. Secondly, our first strike can't assure destruction of all launch pads of Nuclear missiles. If if we manage to, there's always an aerial route.
Our first strike will provide the perfect reason for the other side to put in everything that they've got.
Nobody wants a nuclear war. Nor us, nor the Pakistanis.