Throughout The Lovely Bones, Ruth is continually characterized as one who desires to leave Earth. Susie even describes her as being other-worldly and senses that Ruth has an understanding of greater powers. After her death, Susie continually reflects on that fact that she never got her first kiss, and she deeply regrets having missed the chance. When Ray and Ruth end up together, this, combined with earlier information on the characters, lead the reader to believe that some type of encounter will occur. So when Susie enters Ruth's body for a short time, it is understood because Susie needs to return to settle her regrets while Ruth is happy to trade places into the spirit world.