A Rebbe receives tremendous intellectual powers from the many talmidim and hassidim who look to him for a connection with HaShem. Some people may, out of genuine intent to serve HaShem, seek to be a Rebbe in order to attain this great intellectual level.
The problem is, he explains, it is a double-edged sword. The Rebbe achieves this wonderous intellectual level in exchange for the responsibility for the physical and spiritual wellbeing of his hassidim and talmidim. If he doesn't have the spiritual ability to understand, relate to, and rectify every problem of every one of his followers' souls, then he will suffer the consequences as they fall (because of his ineptitude) and drag him down with them.
Being a Rebbe must come out of the deepest Ahavath Yisrael and nothing else can possibly compare to this. The bittul of someone with true love of his fellow man is the only one who can bear the burden of being a Rebbe. Anything else is hubris and highly dangerous to themselves and those they draw close.