There are many explanations to this question some people will say because we have sinned some say that we are born to suffer strait from the beginning. But what many Buddhist believe to be suffering is merely the mind making assumptions to cause and effect; the wind is cold this cause gives rise to uneasiness in my mind. Samsara (the cycle of exhistence) is not a prison but a tool for wisdom, there are many forms of suffering such as war, sadness, pain,old age, sickness, and finally death. But what we should be mindful of is how this suffering affects us, if we are to scream at the prongs of uneasiness than we fall deeper into what ever suffering we are creating,or is created for us. Instead try to be calm and pray to Lord Buddha for light and calm and meditate upon a single point of concentration then there is serenity, grace,happiness. Even things like obesity, arthritis, cancer etc, can be overcomed with proper concentration of the mind. So dont worry chant your mantras, meditate on a single focus, and be charitable to others. There is happiness after all.