Although the ride may phisically be more or less rough from day to day, sometimes the rider can make a ride more or less enjoyable. Personally, when I am tired, have a headache, or am slightly dizzy from flat rides, coasters are not as enjoyable. The bumps and turns seem more intense.
Sometimes I think people age worse then the rides do. I remember my Dad loving coasters 10 years ago, but now when he rides them, all he talks about is how rough the ride feels, even if it feels fine to me. I'll know it's time to go back to DisneyWorld when I start complaining about my neck pain from smooth steel coasters.