While we're doing Shannon meta, I've been trying to make a post about this for a while but was worried it's too controversial but, screw it: I think the main reason Eddie romanticizes his relationship with Shannon so much is because deep down he feels guilty, and very rightly so, about how he treated her, and so he builds their marriage up to something that it really wasn't in his mind in order to avoid the fact that he hugely fucked it all up. Sorry to Eddie, and it's not totally his fault, because he was a scared kid who was given way too much responsibility way too soon and had no support and awful parents who planted awful ideas of what a man should be into his head, but he dealt with all of that in a way that basically totally ruined Shannon's life. He abandoned her with their child, left her vulnerable to his critical parents and no family of her own to lean on, he never showed her any compassion or support for her fears that she failed Christopher somehow, he never treated her like a partner, he was emotionally cold and distant at best and bordering on cruel to her at worst. She left, but in some ways it's his fault that she had to, because he put her in an otherwise impossible situation. He proved time and time again that he was never going to listen to her or give her desires and needs fair consideration, so what choice did she even have?
Again, I think we all understand why Eddie acted like this. He was literally raised by a father who taught him that a man's place is to provide monetarily for his family while the woman raises the kids, and this is what partnership is. And I think now Eddie knows that's not how a marriage should work, and he probably would do things differently if he had the chance, but the fact remains that what he and Shannon had was not good or healthy for either of them. But if that's the case, then her life was miserable for no reason, without even a loving relationship to justify it, and Eddie just can't deal with that. It's not so much that he puts Shannon on a pedestal as it is he's imagining what it would be like if they actually had the kind of partnership they both deserved, and then substituting that imagination for reality. But we know that he doesn't really believe it, because even when he's half-heartedly trying to get back together with her, the best metaphor he can manage for their love is that it's like falling into molten chocolate that's a little sweet maybe, but mostly it drags you down and nearly drowns you.
And as @livesbetweenpages says in the tags here, all of this is part of the reason the writers had to have Shannon leave, and specifically to leave Christopher and not contact him for so long. Because there's no way we'd be able to sympathize with Eddie if we didn't have some reason to put some blame on Shannon too. She should never have punished Chris for Eddie's mistakes, but of course people are messy and imperfect, and where we end up is with a situation where you can blame both of them but also see where both are coming from. Still, the discrepancy between how their relationship is shown to us (a constant slog of misery and fighting) and how Eddie reframes it in his own mind (being with her was effortless) is always so fascinating and tragic to me. And I think it's important characterization for Eddie, because these are THE fatal flaws for his character, that 1. he resists help, guidance, and partnership over things he feels he should have under control, to the point that it hurts both himself and those close to him (see also: shutting everyone, particularly Buck, out of his decision to move to Texas), 2. when he can't deal with a hard truth, he just pretends it's not the truth at all (see also: Ana).