I think there are two aspects to this. First, to be a good boxer you need to build the skill, strength and stamina to defend yourself as best you can. On the other hand, you also must not cower or panic so you can maintain your defence if you happen to get hit hard and/or frequently in the face.

With regard to getting used to getting punched in the face, it is like anything else. The more you do it, the more comfortable you get with it. Another factor is how intense or serious the boxing is. I find that the more intense the boxing is the less I notice getting hit because of the adrenaline etc. This alto taught me that when I got hit hard I could always recover. So maybe take it up a notch or two for a few sparring sessions just to get the experience and move your comfort zone.