Sympathetic Characters

What makes a character sympathetic? This question sometimes stops me, makes me scratch my head and mutter to myself. I understand that an unsympathetic character is one that I, as a reader or a writer, can't fully relate to. But why? That's the kicker. Is a character less than sympathetic because I don't have anything in common with him? Or maybe I don't have enough information about him--about his emotional state or his circumstances.

I once wrote about a character that was prone to good deeds. Too many good deeds. Hanging out with him made me feel like I was gagging on a sugar cube. This character didn't survive the first draft of that particular novel. Sympathetic characters, it seems to me, should be like the rest of us--flawed, plagued by weaknesses, and prone to bone-headed moves. On hindsight, I bet that if I'd developed the reasons why Mr. Good Deeds behaved as he did, I would have liked him better. If he had a few bad habits or quirks, he'd have been more interesting. Understanding how a character is struggling emotionally as well as physically makes me care more about him. I become sympathetic.

What do you think? What makes a character sympathetic?