In The Outsiders, how do we know Sandy didn't love Soda as much as he loved her?

Sandy and Soda had been a couple for awhile when she got pregnant and went to Florida to have her baby.  As it turns out, Soda wasn't the father - in explaining what had been happening to their brother, Darry tells Ponyboy, "He told me he loved her, but I guess she didn't love him like he thought she did, because it wasn't him...He wanted to marry her anyway, but she just left".  Soda still loved Sandy even when it was evident that she had cheated on him, but Sandy didn't feel the same way about him.  Soda wanted to marry her, but she just disappeared, and his letters to her were "returned unopened" (Chapter 12).