Nice question. People in west find it hard to understand why? Let me answer it is brief and partly.

 First of all, Pakistan is ruled by military and always had nefarious designs against India. Its root are held on the basic fallacy of religion based state and a supremacy based on race(Punjabi in particular and some tribes too). This co-incidently happens to be reason of its internal problems as well. Whenever India surged ahead or Pakistan landed in internal trouble or both -  Pakistan attacked India. This is the history of sub-continent. Sometimes reason sighted were Kashmir, sometimes Muslim Ummah and sometimes pure devilish intent of disintegrating the neighbour. That is history. Alongwith this fact is the hard reality of Musharraf in power. He is a manipulator - he ditched Lahore peace process (google on it), bus diplomacy. He, at one point of time, in Kargil war decided to use nuclear tipped missiles. This is all known to the world. He has support of USA and China. Think about the implication of escalation based on such a proposition. Even if it is misconcept, it still carries potential of war, if Musharraf feels desparately that in order tohold power, he needs to attack India. Add these two together, and answer will be clearer to you.

Hope this helps.

Here is one possible reason: There is a very real danger of our neighbour decending into civil war/anarchy. There could be a huge exodus of refugees to the 'safe heaven'. Who is going to regulate this exodus? Nothing wrong in getting ready for this 'humanitarian' mission.