Depressive symptoms are not simply feeling miserable, but can include feelings of fear, loneliness irritability or lack of concentration, and even sleeplessness.

BBC: Depression may boost heart risk

Other factors involved in loneliness include individuals' levels of social skill, fear of rejection, and self-confidence, Leary said.

CNN: Feeling lonely? Genes might be at fault

All I could do was tell her some of the climbing techniques I'd relied on to help manage the fear and the loneliness, and listen to her anger and bewilderment as she climbed it now herself.

NPR: Waiting and Dreading: Families in War Time