Some of the things that cause wear:

-number of cycles (wheels and motors going around. etc)
-friction
-temperature
-UV
-moisture (not an issue?)
-corrosives
-abrasives
-dirt dust
-mechanical stress/load and resulting fatiguu
-lack of maintenance- lubrication, adjusting clearances

And in many cases, wear will cause more wear

Just looking at the wheels and motors, Spirits 10 inch wheels have rotated about 9500 times in 4.7 miles. (I have a problem here. NASA says all six wheels have rotated 13million times as of sol 779. Not sure where I went wrong with the math? Anyone?)

In contrast wheel bearings on an auto with 24" tires will rotate about 51,000,000 times in 60,000 miles, a distance that's easily reached without maintenance.

But JPL can do maintenance by throwing the rovers in reverse, redistributing the grease in the wheels. Also you'd assume all the bearing surfaces would be finely machined and highly polished.

You could also compare an automotive alternator to the rovers wheel motors. I have 114,000 miles on my original alternator in my truck and its gone 'round and 'round far more than my wheels have. The brushes haven't worn out yet.

What I'm saying is, it seems like Spirit and Opportunity were designed to last far more than 90 days. At least mechanically.