This is psychological, if you send someone 2 merits, the person will have 1 smerits. If someone do you a favour today, will you not remember the person? Meriting someone's post is like you did the person a favour, infact, it is a favour, but yet having quality posts. I hope you see the reason I first implied that it is psychological. What you should comment more about are people sending merits to low quality posts, if you find anyone like that, you can report the person, not reporting someone reciprocating by sending merit to quality posts. You can give merit back to anyone you like, so far the post is of high quality.