This is not Fluffy.

This is not kindergarten; this is not a fluffy movie about middle school.
When faced with a semi-full cafeteria and no one to eat with, upon spotting a small group or an individual sitting alone, it would seem like the natural thing to go up and ask,
"Can I sit here?"
This is not ok.
You will be faced with seconds full of stony silence, before a hesitant smile breaks across their faces and they go, "Um, sure."
Um, sure. Does that sound inviting to you? Not at all my friends, not at all.
It's not that fraternizing and meeting new people is openly discouraged, it's that outside of the first few weeks people find it weird. Everyone develops a rag-tag group, so when some random comes over to sit with you, your first thought it going to be, "Where are your friends?" followed by, "Oh god, they must be a weirdie."
And now, a short anecdote about a friend of mine.
GD was sitting all by her lonesome waiting for the rest of us to join her and this semi-attractive kind-of-awkard Islander comes over and asks to sit with her. GD being particularly articulate said, "Um, sure." And he sat, we met him and awkward conversation ensued. After a short while he began talking to the people sitting behind us, in a friendly manner. Why would someone sit down when they had a perfectly good group of friends already? Because they're a weirdie. Someone totally non-Conformist and sociable. But, of course, that thought is really not all that neighborly. As such, that was the last most of us heard from him. Except the Nicknamer, who just tends to run into people I've termed "weirdies" all over the place. Conclusion: he became entranced by GD's baby blues and convinced himself he had to meet her.
Which throws him slightly out of the category of "weirdie". But only slightly.
I'm going to try and invent a point to all this now...
As weird as it is, people aren't nearly as sociable as you'd think on campus, and they're even more set in their patterns.
Case in point: My particular group (there are A LOT of us) has a designated location you can always find at least some of us at at any meal time. It's like a very easy Where's Waldo.
It's Where's the Cliques.