Tolerance grows with self confidence (more precisely "self-knowledge", as someone suggested it to me).

One could think it's the contrary, the more you trust yourself the more you open your mouth to oppose others,

and that can happen,

but i still think a strong sense of self is a prerequisite for the acceptance of different opinions, so you don't feel shaken in your identity when confronted to someone thinking differently, and because you believe enough in yourself you can accept the differences without getting defensive.

(So intolerant people are probably those not knowing themselves enough)