Use 'blithe' to describe someone's light-hearted positivity, often in a situation where worry is expected. Despite the pouring rain, his blithe demeanor didn't waver.
Pro Tip 2/3
Possibly Naive
'Blithe' can imply a lack of proper thought or consideration, which might seem naive or impractical. Her blithe decision to travel alone surprised everyone.
Pro Tip 3/3
Not Always Positive
Although 'blithe' suggests happiness, it isn't always a compliment and can hint at inappropriate carefreeness. His blithe comments during the funeral were in poor taste.