Naptime-no longer a fun time

Naptime at our house used to be so much fun. Gavin would lay down without any fuss. No problems, no complaining, no fighting, nothing was really said other than a very nice "Good Night for a short nap" Shortly after he would lay down I would and rest as well just for a few minutes or maybe an hour or so if I was lucky. Gavin now fights tooth and nail to lay down. Stomping, yelling, crying, kicking, and carrying on all goes on for a while before the very very tired little boy finally relax enough to rest. I just don't understand why someone so tired argues to lay down for a short while. I would love to lay down for a nap everyday. You get refreshed and have more energy for the rest of the day. Naptime should be allowed for every child and adult, don't you think? I'm not sure why all of the sudden for the struggle to lay down. Any Suggestions to solve this small problem would be helpful!!