Shocks wear out. The nitrogen leaks out, condensation puts water inside, the piston shaft gets pitted, and the seals wear. Most stock motorcycle shocks start feeling funky after just a few years. I replaced the Sachs shock on my Ď07 Vintage after ten years with Ikon 7610s. At the same time I put in Hyperpro springs and fresh oil in my forks. Much, much better.