What if Haruki had killed himself before Sakura died? I mean, he was at that age when a selflessly cheerful girl being so nonchalant about her own death could easily make a boy who fell in love with her unable to cope with the thought of her impending death.
This tearjerker squeezes tears out of anyone in less than 2 hours. But he had to live like that for months, having an actual connection with her. Unbearable, I think.
It would have been rather ironic. Imagine her discovering that the boy she'd been trying so desperately to teach how to live had chosen death over watching her fade away. lol