When you really want to learn, the entire world becomes your teacher. If that is the case, then what is the necessity of a teacher in specific? Can we skip schools, or forget about a guru of spiritual sciences? :)
No, because it is them who can give you the wisdom you need to decipher what you come across in the world and learn from it.
Without them, you might look at a thief and say to yourself, ‘I want to learn stealing’. This is only an example, but there can be mild to major situations when we may make unhealthy choices due to lack of good wisdom. Once our teachers help set our feet in right direction, and eyes in right perspective – learning becomes a beautiful journey!