First of all, changing yourself for another person’s interests is one of the worst offenses in romance and, furthermore, was it not Sandy’s good girl persona that attracted Danny in the first place?
A lot of people like to interpret the couple’s car ride into the sky as a metaphor for their demise, and I would say there is some truth to that – not that they are dead, but their relationship is destined to head that way.
Take it from IMDb’s plot description of Grease which states how the vastly different romantic leads “try to be like each other so they can be together,” a major focal point is that Sandy undergoes a complete transformation into a leather-clad rebel and Danny joins the track team in the hopes of impressing her.