So...It has the Red baron Snoopy WWI fantasy...and that is what you remember. That is the most awesome part of any Charlie Brown movie.
And...beyond that you have a kid making up a story and believing it that would, almost 30 years later, be spoofed in an episode of the Simpsons. So it has staying power.
But really, it's a childhood classic that you love first because it is wonderful and touching, then later because it's nostalgic, and then later because it is a generational tradition.
In other words it stands the test of time like the Godfather and so many other pure classics.