You don't often see these three walk into a room together. But here they are. Hosseini definitely likes to pull at the old heartstrings in this novel. For example, to have Baba spend his life-savings on Amir's wedding (right before he dies) is enough to make a chap weep. The central plot itself, in which Amir rescues Sohrab in order to atone for his sins against Hassan, is full of the chest-swelling feelings we get with a glass of wine and a romantic comedy. The novel is tender, because its characters try their damnedest to do what's right. Though there are some blips along the way, most of the characters end up loving each other. Even though there are some awful betrayals, most of the relationships swell with grand orchestral feelings by the end of the novel. For example: My father is finally proud of me! I love my wife! I love my nephew! I want to buy another one of Hosseini's books!
That said, Hosseini muddies the crystal spring of tender feelings. This isn't Disney, pal. There's plenty of violence and cruelty to go around. Hosseini doesn't shy away from scenes of carnage and warfare, nor does he turn away from some brutal descriptions. (For example, the rape scene.) All this adds a steely edge to the novel's grand emotions.
Plus, Hosseini is in the irony business. For example: Isn't it ironic that Baba also betrayed his friend even though he talks big about honor and principles? Isn't it ironic that Assef finally loses his eye? Isn't it ironic that Amir gets a scar on his lip just like Hassan? Isn't it ironic when Amir, who ends up being Hassan's half-brother, says things like "[t]he curious thing was, I never thought of Hassan and me as friends [...]" (4.4)? The irony adds yet another edge to the story; it gives the more sentimental passages backbone. In the end, The Kite Runner is such a tearjerker, such an emotional tour de force, because it shifts between these three tones.