One of my parishioners has taped to her office wall a drawing done by one of her children. It has written on it (by the teacher) things that the child appreciates about the parent. One of the things in the list says "you are nice to me sometimes". We had quite a laugh about the "sometimes" part. She mentioned to me that it was completely true though -- there were times when she and her husband could not be "nice to her" because they loved her.
I know that I have said this many times, both in writing and in personal communication, that parents cannot be both good parents and friends to their children. Your children do not need you as a friend, they need you as a parent. If parents are going to be responsible to God and fulfill their calling of teaching their children the Catholic faith, then they must realize that being "nice" to their children is not the goal. That does not mean that they should be "mean", but that "niceness" is not actually a trait of good parenting.
When parents are teaching and guiding their children, their goal should be to point them to Christ in a loving manner. Sometimes that means doing things that are not very "nice". I recall once hearing a story about a couple whose children had all left the Catholic faith that they were raised in as soon as they hit 18 years old. The person telling the story said, "it is so hard to believe, they were so nice as parents". I immediately thought, "maybe it was their 'niceness' that actually caused the children to stray from the path." Being a good parent means disciplining the children; it means telling them "no" more than once in a while (and sometimes when "yes" is OK, but "no" is for their better good). Those types of things are not "nice".
Niceness does not allow a parent to tell their children (especially when they are teenagers) that they cannot do something. Niceness does not allow a parent to spend time explaining to a child the problems with a certain behavior of theirs. Niceness may be fine when it comes to how we play a board game together, but it has little to do with parental authority. Now, do not get me wrong; I am not saying that a parent should be harsh and cruel in their instruction of the children. It is the attitude of "niceness", however, that causes a parent to choose the path of least resistance, and avoid any kind of conflict (even if it means that the child will continue in gravely sinful behavior).
This same principle (though there are nuances that are too detailed to outline in this brief post) can be applied to how a priest should minister to his people. Yes, he should be kind to them, and show a gracious attitude. Yet if he is always a "nice" priest then he will never tell them about sin in a homily, and he will avoid speaking to them about the sins that they refuse to repent of in the confessional. Niceness will not allow a priest to be clear about what sin is. It is probably the case that nice priests have bigger congregations, and that they rarely have people complain (or just leave), but the "non-nice" priest, however, can sleep at night because he knows that he spoke to his people (with gentle love) about what they truly need.
I know about a young man who told his Dad recently that he was glad that he had been tough on him. He said that he needed it, and was finally starting to realize the blessing that it was in his life. Although he did not use the term, he was saying that he was thankful that his Dad was not "nice" to him, but instead had showed him love. What a remarkable testimony. Parents should make it their goal to love the children enough to be able to avoid the niceness that leaves a child in his sin. It is certainly not a nice thing to tell a child that he is sinning and must stop, but it is truly loving.