I definitely cook with a view to having leftovers as it is just not worth going to all the trouble of cooking for one person and then having to cook each night.
When I had a partner I probably cooked the same amount but we ate more, but I tend to make too much always in case someone wants more, so generally have always had leftovers whether it be deliberate or not.
I now live by myself,so it really depends on what I am cooking,whether or not I make enough to enjoy another meal! For example......With casseroles and similar,there will always be plenty for either the next day,or for visitors.If I am cooking fresh fish or luscious lamb chops though,I will just cook enough to be enjoyed in the one sitting!
I often have leftovers which I usually freeze because I don't like having the same meal twice in a week, particularly if the meal has been cooked in the slow cooker. However if my teenage grandson is eating, there is NEVER any leftovers as he eats everything including loads of vegetables. We eat European style with all food in covered dishes on the table and everyone helps themselves so there is never any waste as they take whatever they can eat. I never have to leave the table this way.
No not intentionally, Although as you suggest cooking to have leftovers does help when thing about further meals in the week, or freezing day the leftovers for another meal in the weeks to come.
As it’s just the 2 of us at home, I never cook enough for another meal, as I don’t like making the same meal more than once a year. Every recipe I make has to have a date in pencil next to it to it, in a recipe book, or if the recipe is from the computer, I have a list of the recipes I’ve made with dates, in ‘drafts’.