Most of The Sorrows of Young Werther is presented as a collection of letters written by Werther, a young artist of a sensitive and passionate temperament, to his friend Wilhelm. These give an intimate account of his stay in the fictional village of Wahlheim (based on Garbenheim, near Wetzlar), whose peasants have enchanted him with their simple ways. There he meets Charlotte, a beautiful young girl who takes care of her siblings after the death of their mother. Werther falls in love with Charlotte despite knowing beforehand that she is engaged to a man named Albert eleven years her senior.
Despite the pain it causes him, Werther spends the next few months cultivating a close friendship with them both. His pain eventually becomes so great that he is forced to leave Wahlheim for Weimar, where he makes the acquaintance of FrĂ¤ulein von B. He suffers great embarrassment when he forgetfully visits a friend and has to face the normal weekly gathering there of the entire aristocratic set. He is not tolerated and asked to leave since he is not a nobleman. He then returns to Wahlheim, where he suffers still more than before, partly because Charlotte and Albert are now married. Every day becomes a torturing reminder that Charlotte will never be able to requite his love. She, out of pity for her friend and respect for her husband, decides that Werther must not visit her so frequently. He visits her one final time, and they are both overcome with emotion after he recites to her a passage of Ossian.
Even before that incident, Werther had realized that one member of the love triangle â Charlotte, Albert or Werther himself â had to die to resolve the situation. Unable to hurt anyone else or seriously consider murder, Werther sees no other choice but to take his own life. After composing a farewell letter to be found after his death, he writes to Albert asking for his two pistols, on the pretext that he is going "on a journey". Charlotte receives the request with great emotion and sends the pistols. Werther then shoots himself in the head, but does not die until twelve hours later. He is buried under a linden tree that he has mentioned frequently in his letters. The funeral is not attended by any clergy, or by Albert or Charlotte. The book ends with an intimation that Charlotte may die of a broken heart. "I shall say nothing of... Charlotte's grief.... Charlotte's life was despaired of," etc.

Question: Why does Werther decide to kill himself?
Answer: He realizes the only way to end the love triangle is one of them dying.
Alternative answer: he can't kill Charlotte or Albert