They can get slimy and deteriorate from bacteria and other tiny organisms living in vase water. Stem ends can become blocked. They dry out and heal over after being cut and kept dry. Bacteria and other organisms clog stem ends as well. Always be sure to give stem ends a fresh snip before placing them in water.
Roses are sensitive.
There is an ideal growing temperature for roses. After harvest, living processes inside the rose are temperature sensitive. When temperatures become too high, food is used up too quickly and water is lost faster than it is replaced. This can lead to faster deterioration and wilting and even premature death. Recut stems and place in warm water immediately.