In a world where we are bombarded with smiley faces in TV adverts, shop windows, magazines and social media, it would be easy to subliminally receive the message that we are meant to be happy all of the time. Of course, the reality is that negative emotions are very much a part of a human's existence - they evolved to keep us safe (e.g. fear) and to help us be socially 'palatable' to others (e.g. guilt). (The down side of many of these emotions however is that many of the threats out ancestors were exposed to no longer exist but these 'warning' emotions have not evolved away. Also the negative emotions tend to have greater and more sudden impact as our ancestors really needed to take heed when their life was in danger.)
This idea of perpetual happiness can be damaging. If we perceive that everyone else is happy, we might start to think there is something wrong with us when we are feeling sad, worried, scared or angry. Children need to understand that negative emotions are to be expected. They are a part of life and we do not need to panic when we are experiencing them. Healthy emotions, both comfortable and uncomfortable ones, arrive and then disappear or 'flow'. Knowing that emotions generally pass after a while can also help manage them. Validating negative emotions (e.g. 'I would have felt sad too') can also help normalise the negative emotions.
Here is a tool I have used with children to explore the positive and negative feelings they have had over a period of time. In my experience, children tend to complete the graph a bit basically initially but after a while, if you persist, they start to recall what they felt more readily once they know they will be asked to reflect upon their feelings. Children can label the 'ups' and 'downs' with the emotion they felt and with what triggered the emotion. This is an excellent way to help children become more emotionally literate.