Loneliness is a condition where we can only depend ourselves for any strength or mental nourishment. It is a period when we may be completely alone or, although we are with others, we may feel alienated from them and unable to share our unique experiences with any other person at that at that specific moment.
Feeling lonely sometimes is natural but this is also a fact that humans need to socialize. "We need to have a sense of belonging, we need feedback from others, recognition, and a sense that there are others who care about us, understand us, and that we matter to them." Solitude is not always bad. We have all enjoyed being alone- with nature, or music, or books. It is when we feel depression, boredom, and meaninglessness then we have to act. Prolonged loneliness can become fatal if not dealt with as it can deprive a person of his will to live on.