Here are some of the reasons that people give --
Science: “Human development is a continuous process beginning with fertilization and continuing throughout pregnancy, birth, childhood, adolescence, adulthood, and into old age.” (The Endowment for Human Development)
Natural law: “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.” (The Declaration of Independence)
Revelation: “Then God said, ‘Let us make mankind in our image, in our likeness, so that they may rule over the fish in the sea and the birds in the sky, over the livestock and all the wild animals, and over all the creatures that move along the ground.’” (Genesis 1:26)
Does one carry more weight with you than another? Why? And, is that important?