A 20s male is probably pretty high on the list of people to question. I would guess that if you're a US citizen, as they're only concerned with what you're bringing in rather than what you're planning to do or how long you're staying, not looking like a stereotypical foreign terrorist probably doesn't help much.

As a 20s non-white guy I'm never bothered, so maybe I'm just off-base here.

When I was a 20-something white guy (this was pre-9/11), I could count on getting pulled out of line, questioned, and swabbed down in certain airports. I always wrote it off to the fact that I had very long hair and dressed like I was poor, but carried a high-end briefcase.

That stopped happening when I reached my 40s.

I think the 'mostly asleep' might have a good bit to do with it, too - what kind of smuggler isn't at least slightly nervous when crossing customs?

That applies to intentional smuggling, but I'm sure a lot more CBP seizures (by number, if not quantity) are from enforcing agricultural and IP laws that travelers unknowingly or carelessly attempted to violate.

Bringing a citrus fruit from most countries, various other fresh foods, counterfeit goods, etc.