how do we know sandy didn't love soda as much as he loved her?

Sandy becomes pregnant but it turns out Soda was not the father. That might point to Sandy's lack of commitment to Soda. She eventually leaves and returns his letters unopened. 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".

no