Your voice is the only instrument you can permanently break if you practice incorrectly, and you can't go buy another one if you do break it. And you don't know how to practice it correctly and safely until... you've practiced! Catch 22! And no matter how well you "play" the voice, you can only practice so much in a day before you hurt your ability. Violinists and guitar players develop calluses and can take a Tylenol and can keep going, and going, and going, until they get it right. But I've only got so many low E's in me in a day. If I haven't learned what I need to learn by the time I've used them up for the day, too bad about me. Ditto high G's. And you can't decide rent a cruddy voice at first, and then buy a slightly nicer one, and then buy a slightly nicer one. You've just got the one you've got and you have to figure out how to make it work. Mine is great for blending in small choirs and quartets, but it's a small voice. It's never going to be able to cut across an orchestra for opera or giant oratorios. I just don't have the physiology for a big sound. On the other hand, someone with a great big voice can have real difficulty damping down the sound to sing in a group with people like me. They will stick out like a sore thumb. Some people have a big wide vibrato that adds richness to their sound, and some have a fast vibrato that helps their agility for fast runs. You get what you get, to a certain extent, and your teacher can't really borrow your instrument to show you how to get the best possible sound from it. They can only demonstrate on theirs, which is, of course, different.