Okay. I believe Luke is trying to be Nick's "Uncle Pete." Remember when Luke said "don't be like that man" in episode 1 meaning that he subconsciously thought of him as a grown man, but now Uncle Pete is dead and Luke saw how that effected Nick. Now suddenly Luke is saying "I'm worried about that kid." I think that was important since Pete says "I love that stupid kid." Another example that Telltale is hinting that Luke is trying to be Nick's "Uncle Pete" is when Luke says the same thing to Clem that Pete says inside the truck about looking out for Nick. Luke has also been on Nick's case lately (Nick mentions that Luke has been on his case the whole week when Luke and Nick were arguing about shooting Matthew) just like how Uncle Pete was always on Nick's case. Luke thinks that since Nick is so distraught about losing Uncle Pete that he should take that role on with Nick to help him.
That whole knife situation is something that Luke probably thought that was something Pete would've wanted him to do since Luke has known Pete before the apocalypse. Luke is trying to protect Nick, but he's going at it the wrong way. Nick just needs to explain this to Luke.
The thing is, Luke believes that Nick's last anchor was Uncle Pete so he's trying to be just like him, but what he fails to realize is that HE'S Nick's last anchor. The sooner Luke realizes this, the sooner Luke can drop the whole "Uncle Pete" act and just be himself with Nick.
I know that most of you may only feel bad for Nick, but I actually feel bad for both of them. Luke's trying his best to be something that he's not (Uncle Pete). Nick is the victim of Luke trying too hard to protect him. Nick needs his friend Luke, not another "Uncle Pete." Luke just needs to realize this, that's all. Don't forget, Pete was hard on Luke too.