Hinton gives us two contradictory pieces of information: the reveal that Sandy cheated vs her apparent love for Soda.
One reason for this could be so that Ponyboy, and therefore the reader, is blindsided by the cheating. Itβs another way that Ponyboy assumed that Soda had a perfect life when he really didnβt.
Iβve seen theories that Sandy didnβt really love Soda, but I donβt agree with that. I think Ponyβs observations of how much she loved Soda are there for a reason, and not just to throw off the reader.
Iβve talked about this before, how Ponyboy is not an unreliable narrator in this way, that he observes peopleβs behavior just fine. For Ponyboy to pick up on how much Sandy seemed to love Soda, it must have really been visible and genuine. βHer blue eyes glowed when she looked at him.β Thatβs a hard thing to fake. Thatβs the kind of detail you only bring up if you notice it, not the kind of thing you imagine if itβs not there. Itβs the kind of thing you only notice about a person when they donβt realize youβre watching them.
Plus, Ponyboy is really possessive of Soda. See: his feelings about Steve. That, combined with his negative views of greaser girls, make it more likely that Pony would look for reasons to find fault with Sodaβs girlfriend. And nothing is a bigger flaw in Ponyβs eyes then not adoring Soda as much as he does. If he likes Sandy that much, she must have really won him over. And the surest way to win Pony over is to love Soda.
So what does this tell us? Nothing conclusive. I think Hinton purposely gave us a plot that doesnβt add up, that would leave readers wondering what really happened. The point isnβt knowing what really happened with Sandy. The point is Ponyboy learning that everyone has their own life that he canβt assume he knows everything about, including the person closest to him. The point is that whatever happened, itβs much more complicated than it seems.