[sthief09]

I agree that the chance that he flops a set and loses is a BIG factor here.

Odds that he runs into a bigger set, assuming a big pair is out

C(2,1)*C(46,3)/C(48,4) = 15.6%

48 cards, and since it's a big pair he's assumed to see the river. He gets 4 chances, since 1 will be your set card.

A set over set will occur about 12% of the time that he flops a set, assuming there's a 75% chance someone has a big pair. So overally he'll probably win only 85% of the time that he flops a set.

Now if there's TWO big pairs out, the situation gets much worse.