Assumption that life causes suffering is a paralogism. Try applying this logic to another example. Is water the cause of swimming for fish? Is space the cause of movement? Life is just an abstract set of objects which share similar properties, like metabolism for example. Suffering is one of the subjective properties of these objects, so suffering is observed, thus occurs in life. Same as fish are doomed to swim, the humans are to suffer. I would say that suffering is caused by the mechanics of which life is structured. You need to struggle and feel the pain, otherwise you won't figure out what is best for you and your surroundings. Nietzsche said that those who are able to suffer and persevere have a greater capacity for meaning and purpose in life. Suffering is the price you pay for figuring out what is really valuable to your true self, thus is a natural mechanism which occurs in living creatures to help them finding an orientation in the world we live in this reality.