Answer:
Writing thank-you letters is a good idea, but many are hesitant to send them.
Step-by-step explanation:
The first part of the sentence says that 'expressing gratitude improves well-being'. This indicates that saying thank-you or writing thank-you letters is good thing to do. The second part of the sentence says that 'don't seem to walk around giving thanks'. This indicates that people do not do it very often out of fear or hesitance.
Putting this together, this answer is the best option.
I hope this helps! :)