why is their so much emphasis and re-use of the word thirst in this novel? what is Siddhartha thirsty for? why can't he find it?

Thirst can mean many things in this novel. Siddhartha thirsts for wisdom, knowledge, enlightenment...Eventually he must learn not to thirst for "thirst" leads to desire which puts him back into the endless cycle of "samsara". Siddhartha must learn to surrender the many thirsts of life before he can achieve enlightenment.