Why Do People "Feel" Lonely?

Loneliness is a very common, negative emotion in human beings. It is a result from a lack in social relationships compared to what is desired by the person and leaves them feeling quite empty or feeling a strong sense of solitude. It is considered to be very subjective emotion, in the sense that different people react to it differently. Some people are very easy to feel lonely, whereas others may feel happy to spend extended amounts of time without social contact with their relations. The emotion is considered to be a psychological tool used for making it known to the individual that the isolation they're experiencing is undesirable. This then encourages the person to attempt to open social interactions with other people.

The best way to think about loneliness is as a comparison between a person's desired levels of social interaction with their achieved level. It is possible for people to feel lonely even whilst surrounded by people they have close social connections with. For example, people generally feel lonely after losing loved ones, even whilst surrounded by friends and family. Although feelings of loneliness are closely linked to solitude, a person that is on their own isn't always unhappy with the current situation.

There are many reasons as to why a person would experience loneliness. The first, and most obvious, is for the person to undesirably be in solitude. In other words, be on their own when they'd rather be with social relations. Another reason is for a particular relation to be absent whilst other relations are still apparent. Feelings of loneliness have also been closely linked with many social or psychological problems such as depression.

Feelings of loneliness are also very common in people that have recently experienced a loss of a long-term relationship or other relationship that was considered important to the individual. Such examples of this are break-ups, divorces or deaths. This can also be applied to the loss of communication with social groups and falling out with friends. Postpartum depression (the depression a person experiences after the birth of a child) is also linked to feelings of loneliness. Other factors that are usually seen as positive may lead to loneliness such as after getting married, moving to a new home, etc.

This is a good way to get human contact, but a pale shadow of actual contact with others. Loneliness eats at you both emotionally and mentally. It leaves you to feel as if you are the only one even in a room full of people.
How do you make friends? Find others that enjoy the same things or hobbies that you do... Join a book club, or just start hanging out at the library or book store... Go to a gym... Anywhere people do things you enjoy, become a people watcher and seek out those that seem to have similar tastes as you do.

Human beings are social creatures who are designed to interact with others. Feeling lonely is natures way of letting you know that you need to stimulate that part of the brain responsible for communication. I have the gift of the gab. This means I can chat with anyone, even total strangers. Not everybody is like this. In Ireland we have the blarney stone. Its said if you kiss it you will receive the gift of the gab.  So other than travelling to Ireland, why not join a group. I was in the scouts, the order of malta and I took up judo. You will make lots of friends if you do and you will welcome a time when you have some time to yourself.

People feel lonely cause they feel the need to have some one around. They might feel lonely cause they feel like they are missing out on something other people are doing

Yeah I feel lonely when I think other people are doing things without me. But I just ignore it and try n think positive some of the best conversations you have can be with yourself... But about finding people with your hobbies. Not everyone is friendly, however most people are not as social as they used to be. Go figure more social more friendly less social well you get the pic... Anyways if anybody on here is truly feeling lonely you have a friend in me.

I just got out of bed and now and I'm on this site.. 
I was laying there with my lovely boyfriend sleeping next to me yet
feeling so lonely I could cry.!! Just needed to see if there was anyone else out there that feels like me??? I think of my friends and they all seem to just get on and not need people like I do. 
Feel unconnected at times and its lonely...

In my opinion people feel lonely because they feel like they don't belong there

People sometimes feel lonely because they sometimes feel like they have nobody, and that nobody is there for them. But if anybody on here needs a real friend.. I'm here for all yall

But what if some 1 has enough time enough money but still lonely will it be called madness.....
Today I have every thing I even desire for I can get it but still I am lonely have enough friends but still I am lonely...
I don't go to work nor I sit home all day but still I am lonely....
I have many girlfriends but still I am lonely.....

All the answers so far are good one.  Maybe they "feel" lonely because they are lonely.  I am on this site and sitting in the living room with my husband and I am so lonely I could cry.  I am lonely when I go to the store, or running errands.  One can be lonely with or without people around.  We need friends.  My best ones are out of state, so it's limited to phone calls and emails, and a favorite has her house full of children and grandchildren, which makes me feel lonely too.  I find this site an outlet, cause I've made friends, but nothing takes the place of the human contact.

Because they are all by their selves with no one around to speak with.
Well, maybe not counting the little voices, screaming out in pure boredom, being locked up in the little head all the time...Wait till we find a way out, world!

In this fast paced world, one doesn't have time to spend time with the other. This is due to to lack of time for them.

I like everyone's answers. They hit a lot of points about loneliness that make sense to me. If I could be helpful to anyone reading this, I would like to add another thought to all of these: If you are lonely even when surrounded by friends, loved ones, or peers, then maybe your loneliness is calling out for more than social interaction. Maybe your heart needs connection.
We are social creatures. We need other people for things like food, money, a roof over our head, education, medicine, technology, etc. We get those things by being social. We get company and conversation, and time away from solitude by being around other people too. But those interactions don't always answer the deep rooted yearning for connection. If a person feels alone, I am not sure, myself, if making more social connections is going to help. I wonder if that person needs social intimacy.