Because having the right to choose what is best for myself and my body is important every day.
An important factor for women's advancement in society is our ability to control our fertility. Without that, we are trapped by the realities of pregnancy, childbirth and child-rearing; rather than a privilege and a gift, these aspects of being female become a burden.
I see attempts to limit women's reproductive freedom as no more than a gambit to keep women "in their place". To grant a fetus that cannot exist outside the womb greater rights than the woman carrying the fetus is fundamentally incompatible with the function of civil society.
In the words of supreme court justices O'Connor, Kennedy and Souter: "At the heart of liberty is the right to define one's own concept of existence, of meaning, of the universe, and of the mystery of human life."
No one wants to plan an abortion. But the best way to prevent abortions is to prevent unwanted pregnancies: through sex education, improved distribution of birth control, and general empowerment of women to shape our own individual lives.
Among the many reasons that I'm pro-choice is because it is the only option that isn't poisoned with misogyny. I believe that we are responsible for justifying our beliefs, and just as one cannot claim that a given race is inferior to another because "it's what I believe", I don't think one can argue that men can assert control over a woman's uterus -- and future -- because they claim to believe that a fetus is comparable to a human life. That belief is a result of the fact that only women can get pregnant and only women would ever have to carry an unwanted fetus to term.
Can anyone really believe that abortion would even be an issue if men had to face the possibility of giving birth to and raising a child because of one night of failed contraception?
I think Florence Kennedy said it best: "If men could get pregnant, abortion would be a sacrament."