1. Nuclear Weapons
Pakistan has over 130 nuclear warheads capable of targeting every inch of India, there isn't a single place that is safe. These nuclear warheads can be launched by air or land, and make no mistake, India cannot stop them. India's nuclear defences are primitive, in small supply as well as untested. Not to mention if they actually worked India would have invaded Pakistan by now. If you want to know how much damage a single nuclear warhead can do:
https://en.m.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki
Keep in mind, those bombs were manufactured way back in WW2, imagine how much more potent they would be now, and that instead of 2 warheads being used, it would be well over 100. More than enough to destroy India's largest cities.
If these typical nukes aren't scary enough, Pakistan also has tactical nukes that they can and will deploy on the battlefield, giving them the potential to wipe out entire divisions. Not only does it act as a significant force multiplier for Pakistan, but it also shows that Pakistan is very willing to take some dangerous decisions, this alone can be enough of a deterrent.
It's also important to note that unlike India, Pakistan doesn't have a no first use policy, and that Pakistan also has a second strike capability so forget trying to nuke the place first.
2. Development
India is currently going through some heavy development, and things seem to be improving for the nation. If India were to go to war with Pakistan, expect this development to shatter, and for India to instead to regress as a country. Pakistan is more than capable of destroying India's largest cities that are key to its development. No country would ever want to risk shattering their potential for centuries to come.
3. CM-400AKG
India's BrahMos is often touted as another potent weapon to add to India's arsenal, but everyone seems to forget Pakistan's CM-400AKG. The CM-400AKG is the export variant of the Chinese YJ-12, an air launched anti ship cruise missile. It has an operational range of up to 240km and can travel as fast as Mach 5. This bad boy is effectively a hypersonic carrier killer! It entered service in 2012, and can be launched via Pakistan's JF-17's which themselves have a combat radius of over 1,350km, meaning that the missile can be launched from almost 1,600km away.
Of course, sinking a carrier isn't easy, but it is possible and as the the JF-17 gets better, the odds of it actually happening in a war increase significantly.
The actual impact it would have to sink an aircraft carrier would be major, it would throw billions of dollars India spent on the thing down the toilet as well as act as a major psychological blow to lower Indian morale during a war. The mere existence of the CM-400AKG itself could impact Indian morale in a war.
4. India's military
The Indian military, whilst large and powerful with many dangerous tools, have many issues. Many, MANY issues. India's military has a large number of obsolete tanks, armored vehicles, and artillery pieces, not to mention critical shortages of ammunition and air-defense assets, raising serious questions whether India can undertake large-scale military operations at all, let alone whether ongoing defense modernization really is sharply shifting the conventional balance in its favor. Although Indian defense spending has significantly boosted, much of that money has been spent merely replacing obsolete weapons and equipment.
One example of the Indian military's many troubles is its Air Force, the IAF. The IAF has the highest crash rate among Air Forces worldwide. Whilst these crashes can be contributed to technical limitations, it's obvious the lack of skill among IAF pilots also plays a part, since 6 Su-30MKI's have crashed from 2009 to 2015. If technology was the only problem, then the Su-30MKI shouldn't be crashing on an annual basis as it's a new and advanced fighter.
5. Little incentive
Even if you ignore the above reasons, what pain-staking reason must India invade Pakistan for? There is only one reason and even then it's a shoddy one: to combat anti Indian insurgents. But even then, going to war with Pakistan won't stop that, Pakistan will just start doing it even more in an act of retaliation. So going to war with Pakistan wou.ld solve nothing, it would only create lots and lots of problems, some of which I have mentioned in this post.
