Teens meet on a "white line" on the road and begin daily games of imagination that require walking the line. One starts to fall in love and then the other, and we follow the story of how they keep being together for years....before a crucial decision affects both of them.
The start of this is so fluffy and sweet, that it's easy to get too complacent and get hit hard when the darker part comes. I personally want to slap the seme into a coma, for that terrible decision he makes that causes pain for years, but the ending makes up for it. One of the best, sweet, beautiful, metaphysical endings in yaoi/BL...EVER. It's worth crying out of pain to get to the tears of joy.
Life Senjou no Bokura