There's no fixed symptoms for depression. One can even have depression when they are in groups and not lonely. Depression is something which is caused due to pasts or even when someone is tortured by someone so much that they stop even speaking to others.
I saw once during my days of Engineering, when a guy who was too shy to speak to others and how he was been ragged. That ragging was too extreme for someone who is already introvert. Then I saw him leaning on the side of depression.
However, he was one of the to 3 in the college then too I could see him not talking to anyone, getting scared of little things and also resisting to smile even to any jokes whenever he overheard it. One day I saw him talking to himself and when he saw me, he started crying. I helped him to get over his fear and depression. I don't know how I did it but I just stayed with him for hours convincing him to speak.
So symptoms can be of various forms, even smiling without any reason.
One should always consult his/her doctor for such reasons. Clinical psychology helps you overcome such fears