Why do Muslims believe suicides will end up in Hell?

Life is a gift which no one has the right to take but Allah(God) not even you can take your own life. Life is also a test. think of it this way, if you get a test, and instead of sticking through and just trying your best, you get frustrated because the problems are hard and you don't understand them all so you rip it up and throw it away. should the teacher give you an "A' and p you? or should they fail you?