I think the Butcher can explain my reasons for liking Shirou X Rin very well.
Uro: Rin and Shirou mend each other's weaknesses with their own unique strengths, and I think it makes for a very nice relationship.
I also like the fact that Rin is acting more like a romance protagonist than Shirou is in the UBW route. Shirou isn't the one that goes out of his way to ask Rin out on a date, no Rin does that.
Rin is also the one that
(Day 13 UBW VN spoilers)
I also prefer that dynamic.
It's annoying when it's always the male who is at the forefront of a relationship. Though it's not bad, it just gets old after every Jill, Jean and Josie are 'entranced' by the protagonist's charms.
In this case, Shirou and Rin, as the Bastard Butcher said, compliment each other but also it's great to see that they are both rather awkward about their feelings. Even if Rin is the more assertive, she does go all tsun tsun and "ba-baka!" On Shirou when it's him who's taking the lead.
It's a dance between the both of them, and none of them take the lead for long.
Saber and Shirou is just awkward male power fantasy.
Sakura and Shirou are sweet but Sakura is "senpai-kun" boring as hell. Only pro there is that there is a possibility of polygamy with Rider-sama.
Rin doesn't need protection, she's fine on her own. She doesn't need Shirou around him, she wants him to be there.