Why are eBay buyers like this??

I listed a baseball card for $20 which I think is pretty fair since the only sale was $17 over a month ago and the season is about to start. A buyer offers $12 and I counter at $15(again a really fair offer). He counters back at $12 and I decline. A day later he offers $14…. I was tempted to counter at $17 just because he was being the way he was. I counter back at $15 as I don’t think the card should be sold any lower than that. He counters back at $14.50. Like what goes through someone’s head to do this??? I obviously want to sell the card but I would also rather never sell the card than put up with a guy like this lol.