So I’ve been thinking a lot about the opening sequence again, and what they’re setting Stiles up to be.
Lots of people are picking up on Morell steeling Stiles for the battle, same as with Deaton - seeing that Stiles is capable and giving him guidance and tasks, setting him up to fight. But there is Stiles stepping into darkness in the credits, and there is Stiles talking about not being able to do the things that Scott can do, not being able to help.
I think they are harking back massively to Stiles rejecting the bite.
I think that he is feeling the temptation to change, wanting to be stronger, and he knows that option will give him that strength - but he fears what it might do to him too. Like Matt, using a supernatural ability/creature to carry out his wishes. He was a scared 9 year old; then he became a creepy killer with a predeliction for ugly t-shirts. Stiles doesn’t want to hurt anybody, but he does want to be able to fight for his family and friends. He is trying to work out if it is worth the risk.
And I think that Morell is telling him he can have the strength to put it off, and stay human.
Deaton and Morell are telling Stiles to stay human. They are trying to show him how amazing and strong he already is, and can be.