I do all of the cooking in my household, which on average is 5 dinners, 2 breakfasts, even most packed lunches -- desserts, pot-luck obligations, etc.
1. Why do you cook? Is it to save cash or is it recreation? Or something else?
I love it. Also, we do have a frugal lifestyle, so I would probably force myself if I didn't. I have been doing this since my wife and I moved in together -- so about 15 years.
2. Do you plan ahead? Like a weekly meal plan?
I used to -- with so much experience, I can wing it. I make sure to have a lot of choices, a stocked pantry, and I just make stuff up usually. If I find a recipe I want to try, I plan better that week. For baking, I have to plan a little more.
3. What kind of things do you cook usually?
Normal stuff -- basically "Healthy American" -- so two vegetable sides and a little meat, light on starches. I have a lot of Latino and Italian dishes in my repertoire (from my Grandmothers). I will try anything though -- cook out of a classic Indian cookbook sometimes. I use "How to Cook Everything" as a mash-up source -- see how something is basically made (ratios, technique) and take it from there.
Breakfasts are diner-fare usually (omelets, eggs any way, fruited pancakes, etc.).
4. Do you follow any diet? Atkins, Slow Carb etc.
I used to follow Paleo and it influenced my style. I do a lot of slow cooking from that time -- also jerky.
5. Do you have any life hacks, tips to be more productive as a cook?
Cook in bulk. Get a slow cooker. Realize that you will get better at it if you do it a lot. Prep weekly lunches all at once (I do it during the time Sunday dinner is cooking). Use good ingredients. Herbs and spices matter, especially just salt and pepper.