Wilton Barnhardt says you risk irony or you risk sentimentality. Part of this hinges on a really dangerous thing: declaring what you actually find sympathetic. This came obliquely the other day, as I was thinking of rock lyrics used in fiction. When I see a real song quoted in a story, I always think of a warrior covering himself in the skin of a lion as if that will give him the lion's abilities. Music is totally unfair compared to writing novels, in that an audience is only one good chord away from excitement or tears. (Of course music has to earn that reaction, too, but when done well, it happens a LOT faster than with fiction.)
Then there's identification. A writer throws, say, Steely Dan into the story. His character is driving down the freeway, listening to Steely Dan. That's a detail that can only invite distance and mockery on some level. Even if the author loves Steely Dan, and is introducing it in the hopeless idea that it's a universal bonding measure, as if Steely Dan is a pair of frolicking puppies at the world's birthday party, and no one is immune to their charms, even then time and distance will make the reader feel like he or she has to judge the character. "He's driving? And listening to Steely Dan? That BASTARD!"
It is a humbling process, lining up character traits you find sympathetic, and realizing that other people don't feel that way. "He loves to steal from old people! C'mon, don't you just love that? He takes their medication and gives it to deserving school children! He clog dances -- and he's modest about it! Even though he's a champion -- love him!"