We don't know for sure that Sandy didn't love Sodapop as much as he loved her; it is through Ponyboy's narration that we are led to believe this is true. The main reason to agree with this statement is because Sandy admitted to Soda that the baby she was carrying wasn't his. Soda was apparently faithful to Sandy, but Sandy was not. Sandy also refused to answer Soda's letters, returning them to him unopened. We don't know for sure if she had any say in her move to Florida; her parents probably sent her away to have the baby away from Tulsa--to hush things up, as unwanted pregnancies were often done in the 1960s. Soda was certainly torn up about Sandy leaving, and the reader can only wonder how Sandy felt during her exile in Florida.
Because Sandy moved away to Florida while Soda was actually thinking of starting a family with her.
We know because Sandy moved away to Florida.
We know Sandy didn't love Sodapop as much as he loved her because Sandy moved away to Florida