In the series, it's explained that Zangetsu is a part of Ichigo and was suppressing his powers. Since he didn't want Ichigo to become a shinigami, he was practically always hindered Ichigo, as one day he might have to kill him depending on his changes.
Sorry but I read the following part in a wiki. Zangetsu tells Ichigo that the Zanpakuto is physical representation of Ichigo's power, and Ichigo remembers its red ribbon from his friend.
This is where I get a little confused. In one episode, Toshiro and another guy had same dream about the sword and both wielded the sword for a short time. The Soul Society then decided only one can wield him. However, if the Zanpakuto is a part of a shinigami, then how can two people have same power? And how come Toshiro had dreams about wielding the sword, while Ichigo didn't? If it wasn't for Rukia power awaking and Urahara forcing through Kido, Ichigo could never have become a shinigami.
It's like Toshiro's Zanpakuto was given to him by Soul Society because they agreed he was worthy of it after beating the other guy.