People feel lonely because they don't have the required friendships that they need.  Poets described feelings of being isolated, lacking social support, feeling invisible, or feeling that no one around understands what that person is going through.  So in essence, lonely individuals (like everyone else) need friendships, friends who would take notice of them, understand and empathize with their situation, and provide support when needed.  However, lonely individuals seem unable to achieve this required depth of friendship in order to dispel their loneliness.  Some of the reasons given for this include:

Being abused and rejected by others.  In this case, people outside of the lonely individual have treated this person in a rejecting manner.  These include things like being lied to, being made fun of, being abandoned or rejected (by family and friends), being told abusive things like, "you're no good", or "we don't want you around."  In these cases of verbal and psychological assaults, individuals develop defensive walls to protect themselves from this negative environment, often times shutting out both people that can help them as well as those that can harm them.

Being unable to fit in.  Closely linked to the previous idea is another idea of being unable to fit in.  Sometimes lonely individuals feel as if they are "a black pearl in a box of shining jewels."  They feel like a misfit, someone who cannot fit in with the rest of the crowd.  Very often there are feelings of wanting to be like everyone else, to be "normal" instead of standing out and being rejected because of it.  In some cases, the desire may extend to being popular and well-liked, not only fitting in but being looked up to and admired.

Broken heart or missing someone.  Not surprising, in some of the poems I encountered, loneliness was because of a reason break up in a romantic relationship, or just simply missing that special someone in their life.  Sometimes when romantic relationships end, there is a feeling of intense loneliness, and this was especially the case for lonely individuals who experienced a break up with a person that they were still in love with.  Within all of us is a desire to have special people close to us, and when that special someone is torn away from us, these intense feelings of loneliness can occur.