Gerard surmises that Spencer won't tell Brendon for three reasons: 1) Spencer likes Brendon... a lot. 2) Spencer kind of enjoys hearing the conversations Brendon has with Lefty ("No, of course you shouldn't like Alpo. Science Diet is way better for you!") 3) Spencer (and no one else really) wants to see Brendon frown. A frowning Brendon is possibly one of the saddest things on Earth. Causing a frowning Brendon has to be up there with mortal sins (or at least a really really bad venial one).