How did the mariner lighted his hesrt by sharing his grief with the wedding guest? In 'The Rime of the antient mariner'
The mariner is changed by his experiences and is compelled to share the process of this transformation. He wishes others to realise the value of what is around them. He may see his message as a gift to others to protect them from a similar fate and, in this way, can salve his own conscience when he reflects on the terrible events he has experienced.
You can lighten your heart by sharing your grief because it feels good to talk about it. When you are sad, you can be said to have a "heavy" heart. You can lighten it my sharing some of your pain with others, so they can make you feel better.
Part of the Mariner's curse is the obligation to tell and re-tell his story. Maybe we can say that, in a way, he feels unburdened because he is fulfilling his obligation.
Poster #4 is probably right to say that the obligation is a compulsion born out of guilt, but this guilt can be read as a divinely applied guilt which the Mariner has no power to remove. He only has the power to meet the its requirements.
The Ancient Mariner relieves his guilt by telling the story of his killing of the albatross, but his purpose is also to share his new found appreciation for nature and the message that nature loves those who love nature. He is so profoundly changed by this understanding that he feels compelled to share it.
I agree that sometimes, as social beings, we feel the need to speak to others about our sufferings. I believe that this is done in the hopes that we can find one of two things: sympathy and/or empathy. In our sharing, we hope to find others who are sympathetic to our emotions. By having a person tell us everything will be okay, we tend to feel better bout the circumstances we find ourselves in.
On the other hand, our greater hope is that we find a person who can be empathetic to our circumstances. When one finds someone who is empathetic, we have found a person who has been through what we are going through. While sympathy is always good, one who is empathetic has, essentially, been there and done that. Their advice and reassurance tends to make a person feel more hopeful about what is to come.
Outside of that, there is the fact that by sharing an experience with another, we get a sense of relief from the confessional aspect of our conversation.
Speaking to others of one's grief does seem to be a natural human instinct (although it is sometimes suppressed). It would be interesting to know, psychologically, why this is the case. The most obvious motive might be to seek comfort from others. In the case of the mariner, sharing his grief is also a way of confessing his wrongdoings, perhaps to seek forgiveness from God.