Kat is right that some critical analysis should be applied, and some weight could be saved by sharing. Her reasons why we don't share are spot on:
That's how I learned to do it.
I need to be self sufficient. (control freak plus not trusting your partners)
That won't save me much weight (compared with not having control of the stuff and having a partner who fails to bring something)
The difference does not break my back. Kat is right, that is not how we approach saving weight when we take a razor blade instead of a 2 oz knife.
I need my 5" blade. OK, I need a real knife, and I don't want you using it, because you'll lose it, break it, or get it dirty and not clean it.
I have shared stuff in the past, and it sometimes worked out well, sometimes not. We used to have a community cook set and three or four people would eat together. We shared the pots, fire grill (we didn't use stoves) cooking spoons, condiments. We didn't share tents because we didn't take tents. We used tube tents made of plastic. This usually worked out well, but if the person planning meals screwed up, everyone wasn't happy with the food.
Some items are needed for an individual to survive if they get lost, so its not safe to share one item among many peope: fire starter, whistle, map, compass. I'd probably put a knife in that category.
Sometimes its not the need to be self sufficient, its the fact that your partners are unknown quantities and might not bring what they are supposed to, might have crappy versions of their gear, or not even show up.
These days when I share gear, its cooking with a partner, and sharing the stove, fuel, and cookset and food, plus water filter. I only do that if I bring the food, because I don't trust another to do the food. I now have a free standing and a non-freestanding tent that are both light weight and actually big enough for two people. If my wife is on the trip, we share the tent and cook gear, but I carry it all, so its a weight saving for her.
I'd be fine with sharing sun block, first aid kit, toothpaste, tent, cooking gear, water filter, bug spray. Makes total sense.
part of the reluctance to share is due to being a control freak. I want to control the essential items, period. I guess that is a flavor of "self reliant". Do I want to share a knife? "No. You brought a razor blade instead of a knife, so go cut your salami with your razor blade." When someone wants to borrow my lighter, or my drinking cup to use as a measuring cup, when it happens day after day I get tired of it. Its being a burden on others.
I had a couple of scouts not bring lunch food on a week long trip, because they knew that others would bring too much food and would be giving it away. Their mistake was revealing their plan. After I knew their plan, I'd give my dog extra beef jerky whenever they were around. Not sharing when I could have. The flip side of self reliance is not to be a burden on others.
Another reason to not share a tent is "I snore and I don't want to inconvenience you." or "you snore and I want some distance from you." Another is "I stink", or "you stink." Another is "you have food in the tent that the bears will be coming after." Also "after hearing you talk for 10 hours today, I just want some distance from you." another is "I'm going to be peeing into a bottle tonight at 3 AM and I'd like some privacy and to not be bumped." Lots of good reasons to not share a tent.