Have you ever washed your bike with a high pressure sprayer or concentrated garden hose spray? If so, be sure not to direct any water directly at the bearings. The seals on the sides of the bearings aren't water-tight. Once water gets in the bearings, they rust and pretty soon you've got bad bearings.
Hybrid ceramic or full ceramic bearings are one fix for that (at least for the rusting... water can still get in the bearings so you still shouldn't spray them), but they're expensive. I put hybrid ceramic bearings in the rear gears and the wheels... the thing rolls almost too easily now. Backing out of the garage, I've got to keep a finger over the brake lever, because there's a very slight (almost imperceptible) decline right outside the garage, and the bike wants to roll fast once it hits that decline.
Another fix is coating the outside of the bearings (outside the seal) with a coat of super-thick grease, to act as a water repellent.
On my scooter, the right-hand rear swing arm has a bearing that sits deep inside it. A round hunk of rubber cut out of an old tire and fit in that hole blocks any water ingress from the outside.
And you should take your bearings out every couple or few years, rinse the old grease out, inspect them, and regrease or replace as necessary.
