My favorite teacher and I had a favorite saying, "It's easier to teach someone whose glass is empty, than someone whose glass is full."
In other words, take a student who doesn't know terribly much, hasn't been fed any bull about what's right or wrong, what have you. Teach him/her the skill you want him/her to learn.
This statement above, following in this context, is a lot easier than taking a student who is opinionated, knows the basics (or could be some advanced skills), and has heard every cock-and-bull scenario out there, and teaching him/her what you want him/her to learn.
Everyone has their own way of doing things, and those who are newer, or who have not had as much training, are easier to teach than those who have been through a number of classes. Having a "full glass" can be a benefit, but it can be a detriment. That person may want to learn what you have to teach, but their previous training could get in the way of that.
Someone who has an "empty glass" is easily molded, with all techniques that you're trying to teach, ingrained.
I'm kind of wondering whether or not Alex is filling the role of this individual.