He always had a soft spot for Elizabeth as well, I think some of his moments with her are incredibly touching even though they clash. Romano is racist, sexist, homophobic and yet I always found him so compelling on screen, we hadn't really had a decent antagonist character up to that point he arrived. Kerry to a lesser extent, but Kerry is generally a very compassionate person though she has her moments of being infuriating and jobsworth. The ER would collapse without her, let's face it!
I really do miss Doug and Carol once they leave, it always broke my heart that Mark had to go through his illness without support from them, two of his closest friends.
Also, I'm glad I'm not the only one who thinks about Mark's dog. I always imagine that after a few weeks, he realised his schedule was too busy to give the dog the life it deserved, and he found it a good home.
I still wish Mark and Susan had made a go of it, I liked him with Elizabeth but they didn't ever seem quite suited - she was too sparky for him and I think their relationship just couldn't recover from the things that happened to them. The moments with Mark and Susan in S8 are lovely, and while you can see it as them just being close friends, which they are, there's another layer to it. The shot where they focus on Susan's face when the letter is read is devastating. The actress played that so well - not just mourning Mark, but also what might have been.