The choice between becoming a carer and donor seems bizarre to me because wouldn't most clones--especially clones who were in love--opt to become carers so they'd have longer to live? If Kathy and Tommy were in love while they could opt into the carer program, do you think they could have done it to bypass the system?
shareCarers end up becoming donors; that fate is inescapable. Whilst being a carer they witness what will happen to them in due course. Not everyone would want that. In the book it's made clear that every donor is a carer first but some work as a carer for longer than others.
You have to remember that Kathy and Tommy's love was recognised between them only after Tommy had started donating, so he couldn't revert to being a carer as he probably wouldn't have been well enough.
Why problem make? When you no problem have, you don't want to make ...share