Two reasons are usually given for this. The first reason is related to religion. The story is as follows, once upon a time, life always had a clear meaning, brought by God, that is to worship him, but the religious faith has gradually collapsed, not only because God was say) die, but the value you once guaranteed life also follows.
Modern science is the second cause responsible for the current crisis of belief. The scientists say that life is created by the random interactions between chemical compounds and gases, so it makes sense too, but rather in a cruel and limited gloom. For humans and all other organisms, including the amoeba, the meaning of life is the survival and genetics of a species. This sounds right, but it’s also clearly unhelpful and sad.