He and her both had traumatic experiace from young age. Boh incidents involve parents, his parents separated and hers... ummm killed her. Does that explin it for you.
Reply to me if that makes sense.

lol.....i dont think thats enough and not to be rude, but that doesnt even remotely make sense....im sure there's no logical explanation....but no thats not it.....

x_sh_x's answer explains well, but there's a bit more. They're both young children, and they kinda connect that way. It's a horror movie, there ISN'T a logical explanation, it all just happens

she just possed him because she is a ghost.

She wanted to replace him. She wanted Rachel to be HER mother.

@ OneFoggyNight: there was a logical explaination for it, just because it was a horror movie doesn't mean it has no meaning. MarMar_XigLux that's exactly it. Best answer out of all of them, I agree, that's what happened.

No

I don't pretend to be an expert, but I believe Samara, because both of her mothers tried to kill her, and one succeeded, she has never been loved. By anyone, maybe it's her own fault, maybe it isn't, but she sees Rachel as the perfect mother, and so wants to become Aidan, Rachel loves Aidan, because she knows she herself would never be loved, she wants to be loved in any way she can.

....that's what I think.