Like many of you, I always have some students who balk at showing their work even after I insist and we have discussed how problems are going to be getting more complex so let's practice showing our work now while it is still "easy" and how employers are looking for people who can explain their thinking, etc. Eventually, the student will say "but I can do them all in my head" and I'll give that student something to solve that he will need to be able to solve by the end of the school year that there is no way he can solve in his head. It will be too new or have too many parts to it or perhaps it will just have a couple of fractions in it. Very, very few students will resist showing their work after that point, and the few that do will at least not argue with me about it any more.
Way too many of my middle school students have large gaps in the fluency of their basic facts knowledge. This includes place value and multiplication tables. This makes it difficult for them to do fractions, to estimate, to check for reasonableness, and for them to "see" patterns. Many of them are way too calculator dependent. I do allow calculators sometimes and not at other times, although when they get to use calculators, they still need to show all their work, even what they typed in the calculator. I do model strategies and have students practice math facts, especially when I am working with them in small groups or one-on-one, but I haven't found any grand solution to this problem.
A third habit that many middle school students have that drives me crazy is that they have 5 - 10 seconds of perseverance and then they give up. This is not because they are middle school students; kiddos of the same age in other countries will work on something with which they are struggling for up to half an hour before quitting according to research I was reading a couple of summers ago. (I think this was in the book "NurtureShock" by Po Bronson.) We have teeny lessons on "lifeskills" and growth mindset vs. fixed mindset at our school, but it isn't enough. I'm not sure something I do all by myself would be enough, but I'd like to do more.