I love grocery shopping. No, seriously. It's one of my favorite things to do. And I'm an excellent grocery shopper. I make lists, organized by general section of the store, with footnotes and addendums such as, "If they don't have Cinnamon Bears, regular Graham Bears will do." If I'm feeling especially OCD, or if someone else is shopping with me and likely to suggest splitting the list to save time, an asterisk beside an item indicates that I require that brand name; otherwise I do as my mother taught me, and buy off-brand.
Once, when I was a kid, I went shopping with my grandma, and she remarked that she had never met such a thrifty kid in her entire life - I believe while I was selecting a jar of sweet pickle chips for the peanut-butter-and-pickle sandwiches I had requested for lunch. (That's good stuff, man. Trust me. Even better on saltine crackers like super-redneck hors d'oeuvres.)
Now that I'm older, two things drive my buying decisions: Price, and Calories Per Serving.
I have fussier rules within those categories, such as my rule about serving size and cereal. Some cereals try to hide their calorie content by suggesting you eat, say, half a cup. Even if I'm okay with that calorie count doubled, I won't buy it because I think they're being dishonest and I won't support that. No one eats half a cup of cereal and they knew that when they put it on the label. Also, Apple Jacks and Corn Flakes are both 100 calories a cup, and it's hard to compete with either of those.
I think I would make an excellent personal shopper. My friends send me on grocery runs all the time as is; I think some of that is because I'm rarely the one cooking and I'm not that good at baby-minding, but also it's been noted that I get it done faster and better. I could easily parse my overly analytical brain into a useful and entertaining career, if only people would see the wisdom of it.
Because otherwise, I'm just an obsessive compulsive whose Librarian will read this and ask, "Why can't you just go grocery shopping like a normal person?"