The argument that coming into existence is always a harm can be summarized as follows: Both good and bad things happen only to those who exist. However, there is a crucial asymmetry between the good and the bad things. The absence of bad things, such as pain, is good even if there is nobody to enjoy that good, whereas the absence of good things, such as pleasure, is bad only if there is somebody who is deprived of these good things. The implication of this is that the avoidance of the bad by never existing is a real advantage over existence, whereas the loss of certain goods by not existing is not a real disadvantage over never existing.
David Benatar, Better Never to Have Been: The Harm of Coming into Existence