Jack and Alison have known each other for as long as either cares to remember. They have grown into a romanticized version of adulthood, and they're one of those couple that everyone just knows would be perfect for each other – if Jack weren't gay.
That does not explain Jack's reaction when Alison finds what seems to be love after a long string of heartbreaks. His initial skepticism turns venomous as his underlying desire for what she has – emotionally if not physically – leads him astray. Things change, but this is a reality that Jack is not necessarily prepared to accept.
A love story in reverse.