My take on Donald is that as a supporting character, he gives Daryl reason to look outside of his (Daryl's) own self and see how others have similar and much worse problems (i.e. five days being the longest one has been sober, drinking since age nine). Maybe eventually Daryl would slowly own up to his own problems, especially when he had no choice but to do so. Also, Donald may have been a younger version of Daryl. Just my thoughts of course!