As a coach I have to work with riders not only on technical aspects of riding/dressage but also on my student's attitudes toward their horses, themselves, the work they are doing. Everyone knows that the right mind set can help or hinder the learning, training, competition... I am not going to go into psychological analysis but simply give you couple very common examples that stand on opposite sides of the attitude spectrum.

My horse is my friend - This is extremely common set of mind with weekend and pleasure riders. Some competition riders also can end up hindered by such attitude. The problem is that a horse is a large, strong, fast animal that can hurt a human accidentally by simply bumping a person with his head trying to look somewhere. Horses has to be trained to know how to behave around humans, have clear black and white boundaries and confirmation of those rules must happen on the daily basis. However, people end up treating horses like they would treat a friend giving them choices they do not know how to make, trying to be "nice" to them because that is what expected between friends. They worry to be assertive because "my horse will stop loving me", etc. Even if a horse is just a companion he still must behave well for a farrier, a vet, for the owner. This is taught to a horse and reconfirmed on regular basis. My yearling now behaves better then some horses I see around, especially, of owners who do not take any lessons.
If a person has ambitions to ride a horse this creates an absolutely subordinate situation for a horse. It does not mean the horse is abused, it simply means the horse is in a position of an employee, a student at school or a soldier on duty, it is NOT in a position of a friend!

He should know better - This particular attitude is on the other side of the spectrum from the first one. This one is common to hard driven competitors, riders with A type personality and with overconfident people who know without understanding. Horses like any animals who are trained by humans thrive on consistency, clarity, repetition, building the progress in steps. Many horse people think that a horse of a certain age must behave a certain way regardless of that horse's history - what the horse knows, how it was taught, etc. I hear often something like this:"He is 8, he should know better!" Usually said in righteous tone. My questions to that person are Why? You (or, someone) taught him that and confirmed the message on a daily basis? Naive horse knows nothing about how to behave around humans, how to respond to cues. EVERYTHING must be taught and more then that reconfirmed and strengthened with regular handling and training. This includes riding too. Things like standing at a mounting block, moving in a straight line, keeping a steady pace, slowing down and speeding up on cue, turning, staying focused on work, the list can go on...
A lot of time riders are inconsistent in their work, expectations and goals. They may decide to try something new and see it only from end goal perspective not a training development. They make sloppy transitions 80% of the time at home and then get upset during a lesson or competition that their horse made a sloppy transition.
Also, every horse learns differently and takes different amount of time depending on it's personality, conformation, skills of the trainer, frequency of lessons, etc So, things like 30 days of training should produce that and that are very unfair to a horse. A popular spectacle of Cowboy Challenge at Mane Event is a good example of such attitude. And then people actually believe that these horses were started under saddle in front of their eyes and they buy them for big money.

The other examples of wrong attitudes are He always does it! (problematic behavior can be retrained); I only trail ride, I do not need lessons ( trail riding has more potential for accidents then riding in arena on the flat); My horse will never hurt me because he loves me. (horses are not pets); He is still young! (often used phrase to excuse a horse's bad behavior, I heard it about a 6-year old!))
Be mindful of your attitudes, they set up the atmosphere of your handling, training, riding, competing...

Happy riding...