Why did Auggie's dad not want him to wear masks?

In the book Wonder, Auggie has a facial deformity that is very difficult for other people to understand. They react in various ways as soon as they see him that make him feel uncomfortable. Auggie found that be wearing an astronaut's helmet, he could eliminate the intial reaction that people had to his face and he felt more normal. He began to use this 'mask' as a crutch by wearing it constantly.
His father took the helmet and threw it away without telling him much like parents do when a child refuses to give up a childhood security blanket. He realized that without his help, Auggie would be unlikely to give up the mask and start facing the world as his himself on his own.
His dad finally tells him the truth at the end of the book that he "couldn't stand seeing that thing cover your face anymore" and that he loves "this face of yours, Auggie, completely and passionately. And it kind of broke my heart that you were always covering it up." Auggie's dad shows here that he did what he did to give him the confidence he needs to be himself in the world.

In the chapter called "Costumes," Auggie explains that when he was little, he wore a kind of mask—an astronaut helmet—wherever he went. He used this type of disguise to hide his face, which is marked by a deformity. When Auggie was very young, he did not want people to stare at him, and wearing the helmet, even in the summer, allowed him to hide his face. The helmet also allowed him to isolate himself to some degree from the rest of the world. Auggie wore the helmet constantly until he was about seven and had eye surgery, and, following his surgery, he couldn't find the helmet anymore.
It turns out that his dad thew out the helmet without telling Auggie. At the end of the book, his dad tells Auggie, "I couldn't stand seeing that thing cover your face anymore." He goes on to explain, "I love this face of yours, Auggie, completely and passionately. And it kind of broke my heart that you were always covering it up." While Auggie hates his own deformed face, his father loves his son's face. Auggie's dad found it depressing when his son kept wearing the helmet to cover his face, as he loves his son's face and wants his son to feel confident about himself and his looks. By throwing away the helmet, Auggie's dad was attempting to make his son rely less on hiding and instead face the world. He wanted his son to learn to accept the way he looks and to show his face to the world.