When dealing with this subject, it is good to get some things defined. I think there is a big difference between being alone and feeling lonely. Then, there is isolaton. Spending time alone, at least for some people, is a crucial part of maintaining one's sanity. I don't care how many books one has read stating that man is a social animal. Man is also one of the few animals who craves, and even needs, alone time. This is not to say man isn't social, because man is. But, man can withstand longer periods of alone time than most "social animals." Obviously, there are differences among individuals. Some people enjoy long stretches of time alone. There are people who can't stand the thought of being alone for more than five minutes. Most people fall somewhere in the middle.
Now, what makes a person lonely? It's one thing to choose alone time, but quite another to be left alone and not feel like there's anywhere to turn. Probably, one of the biggest contributors to loneliness is a radical change. Death of a loved one is obvious, but even if someone particularly close to you was to move away, there would be a natural void. That void is what most of us would call loneliness. Being alone can be considered a good thing, but loneliness just sucks. I had a friend, many years ago, who's wife had recently died. He told me, "I don't think you realize how powerful loneliness can be." I think he was right. I've felt lonely for short periods, but never had the devastating kinds of loss he had just experienced. True loneliness is a rough thing to get over. True loneliness can lead to despair.
This leads to my third distinction: isolation. If being alone, but not lonely, is a choice; and if loneliness is the overwhelming feeling of being left alone without choice, then isolation can be the worst of the bunch. I believe that isolation can come in two forms-both of which can have drastic consequences. One, we can feel so lonely, that we feel there is no where to turn. We feel abandoned by the world and this can be called isolation. Or, and this may be worse, if we've suffered a recent loss, we may decide to isolate ourselves. If one does this for a few days, it is understandable. If one isolates to the point of not even recognizing their friends, it can be dangerous. True isolation can lead to major depression and, if left untended, something as drastic as suicide.
Yes, loneliness can be a powerful thing. The best thing one can do for oneself is to seek help. For those who already feel lonely; isolated, this may sound harder than it really is. The worst thing one can do for loneliness that is reaching lsolation is to let it fester. Too much alone time can be just as draining on one's mental health as not enough. If anyone feels that alone, they need to find some help - and the sooner the better.