[Very, very calmly:] Because some day you may not be here. You may be injured, or gone, or working on a task that can't be halted, or dead. We all know this is not necessarily a permanent billet. You might be sent back to your home world and he could get left behind.

He needs to be able to function and survive without you. You need to be able to function and survive without him, because it goes both ways.