Thanks for the ask!
For me, the Caryl bond is so special because of the understanding and support they provide for each other.
IMO, they understand each other even better than they understand themselves (in large part because I think they’re both overly critical of themselves). They get each other without any need for explanation (for example, Carol understood why Daryl left with Merle and Daryl understood why Carol killed Karen and David) and without judgment.
They are so incredibly supportive of each other and, actually, I think that’s the big reason why they aren’t canon yet. IMO, they both know how they feel about each other, but they don’t want to push or rush each other (which may be frustrating sometimes, but really makes their bond even more beautiful). Carol knew Daryl wasn’t ready in s3, so, while she flirted with him and teased him, she was mostly there to support him. And Daryl knew Carol wasn’t ready in 5a (I still don’t think she’s ready), so, while he did try to be all chivalrous (with the water jugs), he was mostly trying to support her.
They always seem to put each others’ needs before their own, which is, I think, incredibly important for both of them in a relationship (as they both tend to put themselves last and they should both be with someone who won’t, either intentionally or unintentionally, use that for their own gain). And I think that’s so incredibly rare and beautiful.