Because the old new knee got tired of being the youngest body part.
Because the older newer knee was sick of getting stuck with all the hard work.
Because the old knee got sick and tired.
Because the old knee got bored with being the oldest joint in the corpus.
Because the old new knee insisted.
Because the old old knee had no excuses.
Because symmetry is considered a desirable thing, even when it is not.
Because pain has a way of interfering.
Because pain has a way of interrupting conversations.
Because the patella complained.
Because walking is something you love to do.
Not because of a glorious athletic youth, not on account of marathons run or mountains scaled. Just because. Like someone else’s color blindness and your strabismus. Not because your mother always told you that knees were a woman's ugliest part. Not because you thought otherwise.