Horses are extremely sensitive and know when someone is insecure or confident when riding them and they act according to that. A horse that is extremely easy for one person to ride is not necessarily easy for another person to ride or handle. Horses sense people’s feelings and personalities, and different horses have different skill levels of training and they will act according to how much training they have had. A well trained horse that behaves well for an intermediate rider could give problems to a less experienced rider. Some horses also prefer one person and only ride well for that one person. They can be very attached to their rider and perform well only for that person.
Horses are very smart. A horse can be advertised as a “babysitter” (a good, calm, horse for a beginner) because the owner knows that it is a safe horse for a new or inexperienced rider; but the horse may not act accordingly. Some horses enjoy doing things such as blowing out their stomach when saddling, trying to eat grass while riding, refusing to move, or, running off with their riders. These are usually not bad horses, they just know more about riding than the person riding them and show that skill by doing things that they normally would not get away with. Also, horses can be just like children in many ways. When they get a new rider, they want to test that new rider to see what they can get away with. They know the rules of their last rider, but they may want to test to see if those same rules apply with their new rider. They will often start off with little things to see if they will be reprimanded. If their new rider is not quick to correct this behavior, the horse is likely to think that other rules they are uses to may not apply and they will begin testing in bigger ways.
Then there are horses who are just particular as to who rides them and do not perform as well for another rider, especially if they are attached to their usual rider. Their level of attentiveness probably will not be very high if this is the case, and while they will do what is asked of them, they will not do it as well, giving the horse and rider and inharmonious look. Different horses and different riders are compatible; you would never put an inexperienced horse with an inexperienced rider, this is called over-mounting and is dangerous for both horse and rider because the rider will lose confidence if he has a horse that is unpredictable and that he cannot control. The horse can also be ruined by this; he will pick up bad habits from an inexperienced rider that lets him get away with things because he has no control over the horse. Horses and riders need paired by skill level and temperament. A timid rider needs a calm, easy to handle horse. A flighty, timid horse needs a calm, unfazed rider. Try to avoid horses beyond your skill level and expertise. Learn more as you go.