(((((((((SweetAndTrue)))))))))
I just wanted to say I know how you're feeling. I'm in university too and due to depression and the fact that basically everyone in my year has gone abroad except for me, I don't know most of the people here. It can be very, very lonely.
I think the reason I personally isolate myself is that it's just easier. When I'm feeling low, it takes an incredible amount of energy for me to put on the "I'm fine" mask and pretend everything is all right. When I'm alone, I can burst into tears if I need to, I don't have to worry about keeping up with conversations or being engaging or hearing those terrible voices in my head that tell me what a loser everyone probably thinks I am. The lower my mood, the harder it is for me to engage in conversation because I just can't concentrate on what's going on, and having to smile is just painful.