What a wonderful little story! I think you hit the nail on the head with the reasoning in this. I love Marissa's character, but I also can't see them in a romantic light.
I saw this was your first EE story, but I certainly hope it won't be your last. :-)
Awesome! You've got a great feel for both characters. Though I'm a firm Gary/Brigatti shipper, I too have had my moments of wondering about Gary and Marissa, "How come they never . . .?" But perhaps the fact that the story went this way meant that the real characters got into your head and were telling you that it just wasn't meant to be! ;-) Anyway, good work.