umm...did you ask?

Why do people borrow without asking first? It is an irking action which is quite bothersome to many people, due to the fact that if you were to go use something, it may not be there anymore because someone may have taken it -without asking- or may not have put it back in it's proper place.
On many occurrences i have come home to see my sister on my bed, with my blankets and pillows, when she has her own. She also likes to wear my socks, and takes them without asking, which is equally, if not more, annoying due to the fact that when i am getting ready, i am not able to find one pair of socks because she has worn them all. I have addressed my sister many times about taking my stuff without asking first, probably on a daily basis actually, but to this day she is still wearing my socks. Another situation occurred in the tenth grade, where i had to go to my locker and i had just left my pencil on the table, not thinking anything about it.
When i got back the pencil was gone, which was really just irritating because i was expecting to just get back to work as soon as i got back to class. By the end of class, i found it across the room being used by a group of guys. I simply went over to them and asked where they got it from and if i could have it back. I did end up getting the pencil back, and i know that it is just a pencil, but can't people just ask first?!
I think that people have a tendency to "borrow" objects without asking first because they may not think about it, that if something is left unattended it is just free to take. Or maybe they simply think that the person that the object belongs to simply wont mind if they just take it without asking. Although, in the case of my sister, she just takes/uses my stuff without asking because i tell her not to. After all, what else would younger sisters be good for if not to be annoying, and act like they don't have ears.
Factors that may contribute to this behavior may be obliviousness, due to the fact that people don't think about what the owner might feel or think. I don't think that the person taking is aware that it is bothersome to the person whom they are taking from, because if they did know, then they wouldn't take without asking. People may be dissuaded from committing this action if someone took something of theirs without asking first, but thinking about it, I think that this has happened to everyone multiple times, and people still choose to take, so I'm not sure what the ultimate dissuading factor might be.
At the end of the day, there will always be someone taking, and someone looking for what was taken, because this isn't exactly a perfect world, but would it kill to ask first?!?