An easy answer... makes for more "excitement" in the story. It's a "different" love story. Just an easy answer.
My girlfriend and discussed this issue prior to BD - Bella is begging to become immortal at every turn. And as December
December wrote:we've all been in this fandom so long we've come to take it for granted that Edward and Bella had no other choice. That realistically the only way for them to stay together, not just for eternity but even for the rest of Bella's human life, was for her to become a vampire. That's basically the whole point of NM and EC, isn't it?: to make clear to us that they were totally boxed in. Bella wasn't choosing immortality, per se, but she was choosing Edward -- and she couldn't have one without the other.
We stopped and tried to look at it through 18-year-old eyes... I remember when I was 18 (it was 20 years ago, but I do remember!)... I was MADLY in love with a boy. He was my best friend, my soul-mate. I believed that there was one perfect match for each person in this world - in the universe. We had that first love innocence. We thought we would grow old together (he used to sing to me the Beatles "When I'm 64"). I view Bella's love of Edward through those eyes. (And SM has told us that their love isn't just a crush love, it's the REAL deal.) In those eyes, living forever seems like bliss. And at 18 I didn't want kids. I don't know if I would have realized the sacrifice of not having kids at that point. All I saw was him and me forever. (And into the next life, too.)
SO, is this why Bella and Edward "didn't have a choice?" Maybe. But now I look at their love from a married, mother of two's eyes, and I see a different love. I wouldn't give up my life to live forever - I love my kids, I love watching them grow. It would KILL me if they ever left this life before me. I love my husband, but come on, would I want to spend EVERY MOMENT of forever with him, umm... nope! Do I plan on spending the rest of this life and into the great unknown with him, yes... But we can go out into society and spend time with other people! (I wonder what the divorce rate is among the immortal?!
, you asked why not make Edward human... (I think that was your rhetorical question.) The only thing I can say about that is... It wouldn't "fit." Although SM rewrote the definition of Vampire... I don't think readers would have bought the idea of a Vampire regaining his life and becoming human. I know I wouldn't have. That would have been to "unreal" for me. I honestly thought that Bella was going to remain human before I read BD. I thought that she'd be with Edward, but as a human. Unrealistic, I know... but that is what I thought would happen. Which is why I think the only way for her to become a Vampire was to create the life and death situation.
Okay, I've said a lot here... I hope it adds to the conversation. I just kind of jumped in the middle of it!