Did you every read something you knew was symbolic and KNOW you are right? That was that paragraph in the book for me. I'm 99.9% sure that is the reason Snape did what he did... and that bit of symbolism is actually one of my favorite lines of the book. Snape was
propelled by his love for Lily... not for any love he had of a 'son of Potter's.' It was Lily, that trace of Lily in him that he fought for, and if you think about it, eventually died for.
His dying wish was to see Lily, so since he couldn't do that, he did the next best thing... he looked into her
eyes.
*ish a huge Snape and Lily fan... and loves any and all symbolism*