Edward loved Bella too much to take away her soul... he really believed that he did not have a soul because of what he was and he did not want to take away hers and kill mher chances of going to heaven because as we know he feels he is bound to hell and he does not want that for Bella.
He wants Bella to live a life, the only life she once knew and not change into a monster because she wants to always be with him.
He wants her to enjoy life and keep being who she is. If he was the one encouraging Bella to change then he would have been a little selfish.
Bella on the other hand knows what she wants and she decided that she will not grow old and have a 17 yr old boyfriend (eww what will people think? lol) and die without him... she decided that one lifetime is not enough to be with her lover man lol so they both have their reasons...in my opinion :)