Why aren't people allowed to kill themselves?

is most (if not all) religions says killing youself in going against god's will. The heavens grant the life to a person, the most precious gift of them all that is not returnable, and if you decides to throw it away because of some petty matters (anything aside from sacrificing your life to save another's life is considered petty) it shows you are being rude and going against the heavens. also, life is meant as a tool for human to help other people. if a person is dead, they can't help other people. even if you are disable, ugly or whatever, you can still motivates peoples to do better and appreciate life even more. therefore, by religion people are not allowed to kill themselves. (sacrificing is a different term)