True love is unconditional and expects nothing in return. This kind of pure love is pretty rare, and it's generally found to some degree only in the most committed relationships--such as between parents and their children or in a marriage relationship, as mentioned above. Now, the reality of life is that we're generally selfish and want to be loved in return. That makes our love conditional. (An equivalent is gift-giving. We generally say we give gifts simply because we want to; however, if there is no reciprocity--if we receive nothing in return--we often get resentful.) It's the unconditional nature of love which is so difficult for flawed human beings to master; only if we get close to that standard can we expect to maintain love when it's not reciprocated.
It happens all the time I would think, though I would agree it is difficult for love to grow or even maintain without an answer of love in kind. That strong a human emotion is complicated, and the forms of love are many and varied, from romantic to platonic, from brotherly to motherly.
On that note, love between parent and child/child and parent is sometimes tragically given in only one direction. The same might be true of brother and sisters. In whatever relationship, the longer a person loves without love being returned, I would think, the harder it is to continue the love.
I am reminded of a great scene from the film, "Adaptation." The two brothers are at a pinnacle moment in the film, where death is imminent. As they are hiding, a random moment from their childhood pops into their mind, when Charlie tells Donald about an experience where a girl that Donald was asking out on a date rejected him and then laughed about it after he left. Charlie saw the whole thing from a window and decided to tell Donald, who was smiling as he heard the story. Charlie is puzzled at this reaction and asks why he, Donald, is not angry. Donald has a great line: "Because it wasn't about her. It was mine, that love. I owned it... You are what you love, not what loves you."
At a primary level, all of the previous posts are really accurate. It is extremely difficult to engage in any kind of emotional or romantic love if there is not reciprocity evident. Indeed, from this one must move on and seek to find different and, hopefully, better elements. Yet, on a more fundamental level, the love that we have for someone else or something else is ours, and it is what we "own." Our feelings are our own business. If someone rejects them, perhaps these feelings are not meant for them, but they are still a valid expression of our capacity to love and our capacity for love. Just because they are set aside, and as painful as this might be, this reaction does not denigrate the feelings we have as passionate creatures and powerful beings of heart and mind. Indeed, to love someone is to share in an experience both feel. Yet, if it is something that is rejected, this does not invalidate our experience or our feelings of love. I think that the reception of our love and the experience of having such feelings might be two different things. The fact that we are "what we love" is something that cannot be denied nor taken away, even by another. It is probably this experience that allows us to understand a transcendent notion of love, in that we experience our emotions and revel in them, not letting them take the form of the world around them. In this light, loving someone that doesn't love us back is something that does not hurt as much because, to a certain extent, their rejection proves that the love we have "is not about them." It's what we "own." It might be one of the few things we do possess purely as our own: Subjective emotions.
I am reminded of a great scene from the film, "Adaptation." The two brothers are at a pinnacle moment in the film, where death is imminent. As they are hiding, a random moment from their childhood pops into their mind, when Charlie tells Donald about an experience where a girl that Donald was asking out on a date r
I agree with the first post - romantic love does not work unless both people give equally. If you love someone romantically and he/she does not give the same amount back, you will always be wanting/needing.
On the other hand, if you become a parent one day, you will experience the feeling of unconditionally loving someone who is not yet capable of loving you back. Sure the little 8-20lb child will need you, and maybe even LIKE you, but love is not in a child's vocabulary until sometime around 2 years old, and even then, it is somewhat conditional. But you will devotedly love, give, sacrifice and pour yourself out for that someone for a few years without needing love back.
Then, they will love you for real, for about 8 years. When they hit 13 or 14, they might decide to stop loving you on purpose.
I think, in reference to your question directly, the ONLY way to love someone who doesn't love you back, is in this parental/guardianship situation.
I suppose that this depends on how you define love. I do not believe that you can be in a romantic relationship with someone who does not love you. However, I believe that you can care deeply for someone and want the best for them (I think this is a definition of love) even if they do not love you.
In terms of romantic love, though, I do not think that you can have it if only one person feels it. I think that romantic love is something that has to be shared. It feeds of of things that people do together. If one person does not share this feeling, there will not be the closeness needed for true romantic love.
So I think that you cannot love someone who does not love you (in a romantic sense) for very long.
First you have to learn to love yourself and then you will be able to discern whether it is best to find someone or something else to meet your need to love and be loved. Or, maybe when you start loving yourself this person will see that you are indeed very loveable.