I don't think most of us "become" pro-life, I believe that God makes us this way. Unfortunately, many eventually disavow their need for God and fall prey to the Culture of Death.
To me, that is a whole person change. To go from "I can't judge others" to "It's murder and it's wrong." was a significant change in my heart, mind, and soul.
It is, however, my opinion that we are made pro-life, some of us become "pro-choice", and then we may become pro-life again.
All anecdotal evidence I have points to children being naturally playful and not hostile to other children. Sure, a child can put another child in danger, but not out of a hostile intent. Form this we can extrapolate that the natural human is pro-life.
Two factors, I think, make people pro-abort. One is selfishness, that comes with sexual activity. Adolescents often have sex or aspire to have sex with someone who is available, rather than someone who they want to marry. As the culture becomes more paganized, scenarios of premarital sex become more common and less oriented toward future marriage. A pregnancy then is a serious problem, something that ruins plans for college, etc. so abortion is seen as a backup contraception, something that is an enabling technology of premarital sex. This is the selfish motivation, that some people carry through to adulthood (perhaps, because they never become fully adult).
The second is a misconception of freedom. It would indeed be an ugly thing if, say, women were subject to periodic preganancy checks in order to ensure they did not commit an abortion, or if they were searched for possession of abortifacients. Since the proaborts skillfully extrapolate just that, and since we see a rapidly expanding state, there is a genuine fear that is prolife politics succeed, the state will be empowered once more.
It is important to distinguish between these two motivations. For example, it would be unfair to accuse the second group of selfishness or immaturuty. It would be pointless to engage in a lengthy discussion about social theory and the role of the state with the first group.
Another factor one might mention is hostility to religion. Since people correctly realize that all seriously religious people are pro-life, they see a threat to their secularism. Their argument is that pro-life is a church dogma. Therefore, they argue, the irreligious are free to be proabort. However, I don't think it is a fundamental reason people become pro-abort; it becomes a rationalization for either of the above-mentioned two.