I think that in this moment of Snape’s extreme vulnerability, Dumbledore is moved to tears because he sees just how much pain Snape has been in. Snape’s singular devotion to Lily is tragic because he valued her so much more than himself. I think that like most bullies, Snape lived with a powerful mixture of insecurity and self-loathing. I’m not excusing his behavior, but merely trying to empathize. Snape’s father beat him, and Snape may have coped with that mistreatment like many people do, by telling himself that he deserved it. Then, when Snape met Lily, he would have seen in her all of the human dignity and value that he believed was missing from himself. As a Hogwarts student, Snape tried to puff himself up as the intelligent, ambitious, special Half Blood Prince, to counteract his deep insecurity about himself, but in their fifth year, Lily’s rejection of Snape would have confirmed Snape's deep fear that Lily was too good for someone as fundamentally unworthy as him. With all of this in mind, it’s hard to imagine the depth of pain that Snape must have felt once he started to blame himself for Lily’s death. At that point, I think Snape would have committed suicide without Dumbledore giving him a reason to go on. Snape felt that the only use of his life was to continue the mission that Lily died for - keeping Harry safe and defeating Voldemort. Snape hated living with himself, and he took it out on everyone else. And in this moment when Snape casts a doe patronus, it shows that even after all this time, Snape still hasn’t found a way to forgive himself, love himself, and move on. I was just as touched as Dumbledore when I read this, and yet I still dislike a lot of Snape’s actions. That I can simultaneously feel such empathy and such dislike for the same character is a masterstroke on JK Rowling’s part, and a powerful lesson about humanity.