The "when, where, were, we're" segment of "if the world doesn't end" is one of my favorite moments you've ever written, both lyrically and musically. It sounds great and gives me the feeling of an epiphany when I hear it. How did that come about? And why "when"? Why not "why" or "what"? Something about the sound of "when"?
if i could try to psychoanalyze what might have happened here, i'd say the line starts "here you are, there we were", so i think after saying 2 W words in a row, i think that sets a train in motion where you wanna keep going. And the previous 2 phrases started with "here" and "there", ... so you wanna say "where", and also "when", and of course "where" is naturally followed by "were", and then "were" easily transforms to "we're", and of course everyone loves a fun apostrophe game, and then now you're set up for an echo of the same line in the previous verse in that place (around again), and also fits with the point of the song (the world not ending). This is just my analysis of the line after the fact. when writing it, i'm pretty sure everything from "when" to the end of the line came immediately. Oh right, you were asking why it's not "why" or "what" instead of "when"... and the best thing i can suggest is, try imagining it with those other 2 choices... in my opinion i think that sounds much worse