Ninjutsu is not something that should be used for personal desires.

It is something that should be used when no other choice is available,
for the sake of ones country,
for the sake of one’s lord,
or to escape personal danger.

If one deliberately uses it for the sake of personal desires,
the techniques will indeed fail totally.

Momochi Sandayu – Soke Iga Ryu Karate, Koto Ryu Koppojutsu