Its really simple actually. In the big moments...and in the big matches....Sampras just had the genuine belief and Andre didnt. You could see it in Andres eyes when they were playing.
Andre got his once in a while in the big matches though. Ussually if he was on a hot streak.
I always thought it was a pretty boring matchup when they player. Rafter VS Andre was much more interesting. Neither player was really intimidated by the other. Then again I dont remember anyone really intimidating Rafter once he came into his own. That guy was a warrior.