What are common causes of premature chainring wear? My bike mechanic told me today that my front chainrings are starting to wear out and I should consider replacing it soon (with my next chain replacement). The cassette is still in good condition and does not show any signs of needing replacement. The bike has 3000 to 4000 miles on it and all parts except the chain are original.

The specific reason for the visit to the mechanic was due to the chain dropping to the inside. The drivetrain is all SRAM Rival. There have been no other issues with the drive train. The mechanic seemed to think it was a bit unusual for the front chainring to wear out before the cassette, though it was not unheard of.

Are there any common causes (ex: lack of specific maintenance?) that would cause the chainring to wear out so much faster than the cassette?
Why do you wear chainrings quickly?
Cross Chaining - If you tend to be in the big ring on the back and the big ring on the front at the same time, you're probably doing it wrong. Same with running the small ring on the front with the small ring on the back.  This can cause uneven wear on the front chainrings due to the awkward angles the chain must bend. This can wear the sides of the front chainring teeth and make them more pointy.
You are riding dirty or don't lube up first- If you ride offroad especially, your chain can get dirty. If your chain is not lubed properly, it can cause extra friction. Keep the chain clean or it will work like sandpaper, wearing down your chainrings and make sure you use the proper type of lube for your environment. Chain wear tends to deform the inside surface of the gap between teeth, making an uneven 'U' shape between teeth.
Riding with an already "stretched" chain - If your chain is worn, It has a different shape, which is going to cause extra wear on your chainrings. If it's worn too much, you can only really replace it.  Running with a "stretched" chain is probably worse than running with a worn chainring.

If it's a pretty new bike and the chain is well maintained, I'd probably blame it on cross chaining. Be aware of your riding style and make sure you're not overextending the range of your front chainrings.