Affection thrives in a positive atmosphere. Parents who are happy with themselves radiate positive "vibes" within the home. Children pick up on these vibrations and feel good simply by being in that environment. Similarly, parents who are unhappy with themselves tend to send out vibrations that reflect their emotional state, which children also pick up on. Therefore, we should strive to create the happiest home possible.
However, it's wrong to think that our own imperfections prevent us from doing what's best for our children. Smiling at a child when you're not perfectly happy is perfectly fine. An educator's role is to always maintain a positive attitude for the benefit of their children.
This doesn't mean lying. It simply means there are things that children (each according to their age and maturity) aren't able to fully understand, so sometimes it's preferable, for their well-being, to withhold certain details of the situation. Imagine if an adult is outraged by the injustice they see in society or a war situation: how will a child feel, seeing the adult distressed and not understanding why... or seeing the news reports about crime and not understanding why...