Not 100% sure where I should start on this since there's so much to say. First off, I'm sort of seeing this as a "nature vs nurture" type of debate to start. Are we born into who we are merely through genetics, are we developed into our person through life experiences, or both?
Evidence has shown that both your genetic predispositions and social upbringing play a role in personal development and that is what I agree with. This coming back to the question of whether the circumstances of our birth being irrelevant though bring up a whole new area. Absolutely the circumstances of your birth play a role in who you are - your location, social/wealth class, race, etc. all play major roles. This in itself is included among one of the major political debates in the United States about class warfare and the notion that "the American dream is possible for everyone." Statements like that claim that no matter who you are, if you set your mind to it and work hard and fight through your circumstances, no matter how tough, you can become successful. This I find blatantly false while, yes, it is possible for some to overcome their hardships, the majority don't have the opportunity due to the circumstances of their birth.
If a man is born in the slums of a worn out city with a family of minimum income and on welfare, with public schools way below average, and with poverty surrounding the entire area leading to drugs, crime, gangs, etc., how does he have the same opportunity and ability to become successful and "happy" that a man born into the complete opposite has? Some claim that if people born in bad conditions work hard and get a good education, they're able to grow and achieve their dreams and become happy, in spite of the conditions they grew up in and were born into. But, if a person has a poor education system around them with no money to move to an area they can afford with better schools and are forced to stay in a poor area full of crime and poverty, then how do they have the ability to succeed to the same extent as someone who didn't have these hardship imposed on them? Just the fact that the debate is going on about this possibility shows that the circumstances of one's birth does definitely play a role in one's life and the person one becomes.