The causes of loneliness are many:
Death - of a loved one can decimate a person with loss, grief, and loneliness. Depression often sets in which only acerbates the loneliness.
Aging - as we get older, we often lose friends and relatives. Our circle of people we know and cherish gets smaller, and we can become isolated.
Divorce - many times one no longer can keep the same friends one had when they were married. It can be a major lonely adjustment getting used to no longer having someone to come home too.
Not Fitting in - can often start in high school or earlier. From this time period, one can become accustomed to or expect others to reject them; setting oneself up for loneliness.
Social Media/Cell Phones - relying on social media and texting for companionship is not only superficial, but can be damaging and have long lasting negative affects.