It is far more enjoyable to ride a horse that trusts you rather than one who does not. But getting a horse to trust you is a difficult task, especially if the horse has a history of past abuse.
It is indeed possible to befriend a horse who is shy and reticent but does not have a history of past abuse. Make sure that you approach the horse from the left. And always remember to show him what you have in your hands even if it is a hoof pick, meant to be used on him.
The second thing to remember is always to wear tight clothes. Loose clothing flaps in the breeze and may unnerve you horse. And once you have scared him, it is difficult to make him believe you. Its hardly comfortable both for the owner as well as the horse, if there is an element of fear and mistrust between the two of you.
To put it simply you should not make the horse do any activity, which he does not feel confident to do. The horse has to have full confidence on you, when you ask him to jump or trot down an unfamiliar path. If you ask him to take a jump and he fails to do so, he will lose the confidence he has in you instantly. He will be frightened to take any command from you the next time....