Merit comes as a result of hard work, is the reward for the thorough job you have done in learning, reading and understanding about a particular topic or context and you are able to impact that on others, whoever is impressed by your taught and understand through your post ,gives you merit for your findings and job well done because you are impacting the knowledge you have got on others which helps to improve their ideas .
Merit isn't like that to begin with because merit is implemented because there are a lot of spammers and rank up since the requirements to rank up before is determined by activity points so in order to prevent that or at least lessen the amount of spam posts or low quality post and also to prevent from ranking up to legendary because of bounty campaigns. So for a member to rank up is to post good/high quality posts and wait for someone to send you merits if they deem it worthy.