Hi rdgrad15,
This happens quite a bit in my university environment also, where there's a melting pot of people, from the completely considerate to the downright intrusive. I try not to be too finicky about things involving personal space, but I do get uncomfortable if a stranger stands too close or someone sits next to me with plenty of other seats around. Why this happened to me the other day in my class. Lol This classmate who has often tried to engage me and ask me questions during lecture asked if she could sit next to me with several seats available in our row, and I just plain told her to sit somewhere else so I could have extra space to multi-task between lecture and a tough final project. Other times when people sit next to me without permission in common areas, I get up and move to a more uncrowded location, no if's and's or but's about it.
You have every right to desire plenty of personal space in common areas, and in your case, you already do a good job respecting those same desires in others.