Step Wise Step

We've been best friends for years and vowed to be so forever more, but he suddenly said, "I don't want to be your friend any more!" I said, "Does that mean we can't be together forever?" To which he replied, "Well, there is another way..." But what is this other way? "I don't get it, but as long as he stays by my side, it's all good!" Or so I naively thought...

Source: Baka Updates Manga