There's a scene in the bloopers to Firefly of Nathan running counter clockwise to a rotating camera during a serious scene in "The Message" so that he can show up in every shot for the character's reaction. This continues until the entire crew breaks down into laughter. The heartwarming part? Allegedly, this scene took place right after the cast learned that Firefly was canceled and Nathan was doing it to keep their spirits up.
Nathan himself happily reports that he has a yearly cook out with the cast of Firefly at his place. You can't get any more heartwarming than that.
The stories about Nathan and his first cat are pretty damn heartwarming, especially since the cat came with the house he bought and he didn't really care for cats until he met her.
For his birthday in 2012, Nathan tweeted his followers saying that if they wanted to honor his birthday, they should donate to a charity that provides clean water for children in poverty. Seriously, the man has a heart of gold.
And now that he's done this again in 2013 and 2014, it's safe to say that he's made this an annual tradition, which makes it all the more heartwarming.