He has decided that he wants to be with you. Even though there is no halter or rope on him, you have drawn him right to you, by his own free will. Softness will become even more important when the horse is ridden. If he only comes part way in and then stops, or if he does not come in at all, then he is made to move again. This process is repeated until he learns that the only way he may rest is to face you and come in, or hook on.
Some horses are easier than others to hook on.
A horse that resists will accept discomfort and keep working out there on the fence in his attempts to ignore you. That is his choice. If he has to work up a sweat and get real bothered before he starts hooking on, let him. A horse that is curious or trusting may come all the way in the first time. Others come only part way in at first, or come in but do not stay.