I completly forgot to mention Rose's speech about how lucky humans were that they got to change, and how gems were only supposed to do what they were made to do.

I don't know the whole speech, but i think that was the gist of it.

Hope this helps spark some ideas, or something.

I feel like Rose Quartz turned into to Steven so she would cease to exist and forget all of the evil she's done. Like shattering Pink Diamond or lying to her friends.

Maybe is was both of those reasons?

And she didn't shatter Pink Diamond. She IS pink diamond. There's a Crap-load of proof for it.

It doesn't matter which way you look at that Rose is PD theory. I truly think that Rose loved humans and humanity enough to sacrifice herself, and if that meant she gets to grow up through Steven, then that's what it is. I think her motivation was purely out of love for humanity. CN has a good way of making it sound that Rose was some sort of Gen Jesus. She gave up herself for Steven because she loved humanity so much more than being gem.

Why would she turn into Pteven if her pure motive was love for humanity? Wasn't it her love for Greg and just humans (AND their change) in general?

It was in part that, yes. We do notice, however that in the episode where Greg tells Connie and Steven about him trying to fuse with Rose (and his failure) we hear that Garnet/ one of the gems (dont remember, sorry) state that they 'like this one'. She's had other lovers, apparently and not just Greg. We do find that Rose thrived most in the episode "Greg babysits" (or whatever it's called) and loved the idea that humans grow up and that making a baby would be spectacular. 

"We do find that Rose thrived most in the episode "Greg the babysiter " and loved the idea that humans grow up and that making a baby would be spectacular."

Not just making a baby, BEING a baby! That right there was my point the whole time. =P

(I would have quoted you again, but that'd be 3 many quotes 5 me.)

(Also, the 'other gems' name is Amethyst )