In a speech before the Polish diet Demetrius asserts his claim to the throne of the czars. He hopes for assistance from Poland. He declares that he is the son of Ivan the Terrible and was not, as reputed, murdered in 1591 as a child, but raised in a cloister and that he afterward was in the service of the Prince of Sendomir. He asserts that he is Czar Demetrius. His impressive speech convinces both the diet and the king. Although a formal resolution is not passed by the diet because of a veto by Prince Sapiehas, Poland goes into battle against Moscow. The Poles desire to oust Boris Godunov with the help of the upstart Demetrius. The forceful spirit behind the attempt is Demetrius' fiancĂŠe Marina, Mnischek's daughter and Czar Ivan's widow. The latter has been banned to a cloister by Godunov and has for years been grieving for her allegedly murdered son when she receives the news that Demetrius is alive, after all.
Schiller only indicated the course of the further action. Boris hears about the successes of Demetrius and commits suicide with poison. The new czar is a benevolent ruler until he discovers that his claim to the throne is not legitimate. He is not Ivan's son, but was merely used as a tool by the faction of Godunov opponents. When his mother Marfa is supposed to provide identification, she does not recognize him. Despite the lacking legitimation, Demetrius requests her to recognize him as her son. But Marfa follows her conscience and refuses.

Question: Why does Boris commit suicide?
Answer: Boris heard about the successes of Demetrius.
Alternative answer: because of Demetrius's successes