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Blithe
Positive Vibe
Use 'blithe' to describe someone's light-hearted positivity, often in a situation where worry is expected. 
Possibly Naive
'Blithe' can imply a lack of proper thought or consideration, which might seem naive or impractical. 
Not Always Positive
Although 'blithe' suggests happiness, it isn't always a compliment and can hint at inappropriate carefreeness. 