Originally Posted by
Deathhappens
Well, for one, Shirou is a magus, which means he's one of a very few people she can actually let her "honor student" persona drop around. Ignoring Shinji (who, while "in the know" isn't really a magus) the only other one of those in the entire city is Kirei.
Other than that, their characters are actually fairly compatible in general- they're both traditionalists, they both like taking care of people (even if Rin generally doesn't admit it), Shirou is a house-husband while Rin is a career lady...