It sounds obvious, but toddlers are happiest when they feel secure. According to Dr. Tovah Klein, "a little empathy and recognition go a long way . . . and helps toddlers understand what they are feeling by labeling the emotion." So, as Dr. Klein explained at the lecture I attended, when you run out of Cheerios, the immediate reaction may be to run out and buy more Cheerios. But if you commiserate with them, you'll show them that they can survive the situation. By helping them through adversity, you show them that they can handle it and you ultimately teach them resilience.
Source: Flickr user jamiecampbell