Kakashi doesn't smile, a crucial part of their character's is that they let their emotions eat them. Kakashi was borderline suicidal and still doesn't have a lot of self-worth whilst Itachi admitted that he failed in everything he set out to do.
The answer in my opinion is Jiraiya. Never let rejection of the woman he loved, serving in 2 world wars, betrayal of his teammate, death of his master, death of his students, being led to believe he was the child of prophecy and dying a horrible brutal death at the hands of kids that he saved.
Never stopped him smiling or changed his goofy old self, right until the very end.