Think about watching a child play. See how absorbed they are in what they are doing? That's a good example of what mindfulness is. It is being totally absorbed in the "now".
This seems to be where New Age and romantic ideas about childhood take over. Yes, to be in a state of fascination feels good – maybe because to be in that state requires a sense of security and safety – it is unalloyed with the less pleasant emotions. We adults...our emotions tend to be more alloyed. Which is not to say we don’t experience positive emotions – indeed humans have a positive bias– but as we get older, our emotions get, well, more complicated.
Odd, being “totally absorbed” seems contrary to both “being aware of attending” (parallel awareness) and observing. Perhaps the idea is that there’s no task-unrelated thought (i.e., “mind wandering”) going on, no back-and-forth between the sensory/motor task world and the world that is its own world. Hence, no need for effortful redirection of attention as needed so not to screw up the current task-at-hand (like, say, driving). As many cognitive scientists have pointed out, people often find the experience of mental effort unpleasant.
Something else to think about: as Andy Clark likes to say, the brain is a “prediction machine”. The brain doesn’t live in the “now” – anticipation is the name of the game, whether we’re turning a doorknob, watching a butterfly, or drinking a glass of water. It’s only when we're stopped in our tracks that we realize we were looking down the road we were traveling.
Andy Clark (2016) Surfing Uncertainty: Prediction, Action, and the Embodied Mind Oxford Scholarship Online ISBN-13: 9780190217013. DOI: 10.1093/acprof:oso/9780190217013.001.0001