I don't know the definition of cooking, but I'll try to come up with something. Let's see... Something is cooked when:
1. It's been heated with or without water or fat to a degree where it changes its colour, texture or smell.
2. The ingredients have come from at least two different points in the shop.
3. At least half of the dish has been manipulated (cut, crushed, mashed, shaped, broken etc) by you with or wihout utensils prior to the heating. 

I like cooking and I cook whenever I feel like it. And it's kind of on and off. For example All I've made since Christmas is a cake and a big pot of spaghetti and meat. 

That "no one cooks anymore" is kind of an overstatement. Yes, most people are too immersed in the need to work and don't finde enough joy in cooking to do it in their free time. And many think "Why bother slaving over the fire like a caveman, when I can buy it from the store - a finished, better and tastier product"? (That last one has about as much merit as geocentrism.) However, even excluding retired folks, there are still many people whose jobs (or lack thereof) leave them with enough spare time to cook and I'm pretty sure most of them will. It's a nice hobby. It's healthier. It's usually cheaper. For boys there's also the added benefit of having one more thing to impress girls with. You know, I've always thought in general boys make better chefs. I think we're more inclined to say "Yeah, I know that's not how the recipe goes, but let's see what will happen."

Anyway, viva la cooking! Because when you throw the cookbook away and start relying on microwave-ready fork-included meals, your available dishes drop from a few hundred to a few dozen. At most.