There seems to be one main reason for 'passing' in this society and that would be for survival. Survival in most southern towns and some northern areas to escape the lynchings which seemed to be an excellent pass time for whites in those areas. It was a way to survive in a society that was very hostile to those considered outsiders, inferiors: though these people/inferiors were living in the same towns having the same needs it did not matter. It allowed you to make a living.
Passing helped the African-American community but this had to be done discreetly: giving funds to churches aiding your family if you had not turned your back completely on away from them.
Clare's motivation was not for getting passed the lynching mob or even hate from the whites she was always able to go into the white community even as a young person.
Her reason for passing was from escaping a harsh childhood and escape from the treatment she received from the African-American community. Clare created around herself a shield of protection: a nonchalant, what will be will be, dismiss it if it did not have any importance to me at the time. This was the type of attitude she had perfected it and could be said a bitter attitude it made her a stronger person but also a lonely one also. Clare was, as others who 'passed', was unable to let her guard down. They would be killed, she would show or turn out to have feelings.
Irene's passing was one of convenience. She wanted and liked the opportunity to"play white" when the mood struck her or when something arose in the family that warranted her going into the white community. It was not a matter of (needed) survival for her.