Food "sharing"

Food up North is not cheap compared to down South. Items seem to vary from 1$ to 5$ more each than they would be down south, with heavy things (jars) and perishables such as fruits and veggies generally being among the most expensive. Here are a few items listed from one of my receipts in case anyone wants to compare:

Bananas - 3.99/kg

Tomatoes - 8.19 - 9.99/kg (different kinds)

Avocado - 2.45 each

Grapes - 10.85/kg

pkg 3 bell peppers - 10.89

1 pkg celery hearts - 5.09

pkg 5 bagels - 5.99

pkg 6 soft tortillas - 4.99

a dozen eggs - 3.99 / 5.99 (organic)

philly cream cheese small tub - 5.95

The few dollars extra really adds up quickly... one bag of groceries seems to be 50$ easily.

That said, even though it's more expensive I still enjoy sharing food... having people over for dinner, potlucks etc. I'm not a fan of cooking for myself so if there is someone else home I will generally ask if they'd like to have some of whatever I am cooking. I have invited my room mates for dinner twice now. The first time one said she'd show up and simply didn't (she's only home once a week anyway so that didn't surprise me all that much), and the other "fell asleep in the bathtub" and missed it. She didn't hesitate to help herself to the two pieces of fish left when I had gone out for the night, however! The next time she was at home when I was cooking and I invited her again, 5 minutes before I served dinner she confirmed she'd come down, and again she did not show up at all! And again she helped herself to leftovers. I am left assuming there might be something cultural I'm missing here?

Then there's the case of the vanishing food! I'm not sure if she thinks I don't notice or if by offering dinner those few times I've entered into some kind of "food sharing" circle unknowingly? At first I almost didn't notice, she's kind of sneaky like with the dishes... sliding extras into the pile to be washed but never actually washing any. It started with an orange or two (although clearly the peels were in the garbage)... peanut butter (peanut butter knives that weren't mine)... a few eggs... then within three days I found 3/6 of my tortillas, a can of tuna, and an ENTIRE package of bagels, that I had carefully cut in half and frozen in individual ziplocks had all completely disappeared! She has a good paying job that she can afford to go out and drink and for dinner (also insanely expensive here) which is something I don't even do, and she isn't paying rent at the moment so I don't think it's financial strain... I'm completely confused! She never says a thing about borrowing food and she hasn't replaced anything. Is this another case of food sharing and am I to assume that I can help myself to hers? (Not that I would be tempted based on what little she has in the fridge / freezer!)

I guess I only really enjoy sharing food when I feel that I am "offering" it or when things are somehow mutually beneficial. If at least she did the dishes and helped clean up ever I don't think I would feel quite so uncomfortable about the situation. I'm definitely house cleaner as well. For now I guess I'm going to stick to making things that she doesn't help herself to or I will never end up getting to eat any of the food I buy! So my primary diet until her new place is ready for her to move into: quinoi, vegetables, hummus, veggie burgers and beans. :)