[TokuChris]

The writers were trying to express that Jayden was leaving because the team had grown to care for him and would be more willing to rush to his aid than to Lauren's aid. And that can't happen ("mistakes are bound to happen" - Jayden) because Lauren has mastered the sealing power to defeat Xandred, she has to be kept alive long enough to seal away Xandred for good.

Jayden's presence would only cause the team to be divided if he was in trouble as they'd rush to protect him ... but Lauren is the one they need to protect.

However the writers at the very end made it more of a "I am Jayden. I must go. Because I must go." Keeping true to his stick-up-the-rear character-type, but really defeated the whole "THE REASON I AM LEAVING IS SO YOU WON'T PROTECT ME OVER MY SISTER" idea.