1 Puberty isn't yet completed
Each teen goes through puberty at a different stage of his life. During puberty the voice starts getting deeper as testosterone enlarges the larynx and vocal chords.
2 Low testosterone levels
Low levels of testosterone can prevent a man from developing a deep voice.
3 You feel anxious around people
Emotions such as anxiety can make a voice high pitched. If you always experience anxiety around others then you might not be talking with your real voice.
4 You speak fast
When a person speaks fast his voice can sound less deep. Speaking slowly can make the voice sound deeper. (See How to have a deep voice?)
5 You are speaking with close friends
Studies have found that a person's voice pitch changes according to the people they are talking to. It was found that a man's voice might sound deeper when he speaks to a woman he finds attractive and less deep when speaking with intimate friends.
6 Too much alcohol
Studies have shown that too much alcohol consumption can lower testosterone levels. This could in turn lead to a high pitched voice. (See How does it feel like to have high levels of testosterone?)
7 You are still young
Even after puberty ends a man's voice still become a little deeper later on. By the end of twenties the voice stabilizes then it can deepen again after the age of 45.