Look how cute Saionji was as a kid, I wonder what happened to him when he grew up. :P
Lack of faith in pretty much everything and anything makes people grow up mean and bitter. Realizing that nothing eternal exists is the same as realizing one’s mortality. That along with the fact he began to see Touga as untrustworthy and thus also lost faith in the friendship they had. Though of course, Saionji never TRULY gave up on the idea of eternity. Which is why he became so obsessed with it once he finally saw the chance to obtain it through the dueling games.
Aside from that, yeah, adorable Saionji is adorable. ; v ;