I have chosen it because that is what I believe.
Everybody choses somewhere along the spectrum from conception to birth where they believe an embryo or fetus becomes a person. I believe partial birth abortion is a horrific act of murder. Then the logic goes the same for any late term abortion. The age of viability is constantly changing with advances in medicine.
On the other end some believe contraception is a sin. And a few believe a man needlessly spilling his seed for no good reason is also a sin.
I believe what makes man a being that is totally different from all others is his mind. Before there are nerve cells there is no mind.
It is common for families to shut off life supporting machines if a patient is diagnosed as brain dead. The persons life is deemed to have ended when brain function stopped. Does it not follow that a human being's life starts with brain life?
Neither is there a mind with only one brain cell.
You are simply applying moral relativism to justify your arbitrary point. There is a logical position to take: Conception. At that point, a fundamental change occurs. No other point in life has such profound changes, not even birth. Sure, we change, but from one moment to the next we are not fundamentally different.