Growing up, Sephiroth was lead to believe that he didn't have parents. At least, parents that wanted him. Otherwise why would they give him away?

Shinra and it's scientists were all he knew. From a young age he was forced to endure experiment after experiment. Whether it be a mental one or a physical one. Leading him to complain about something that he doesn't remember at this point, because now he had you. A scientist who was willing to stick around and take care of him.

His blessing.