To dream that you became a volunteer denotes to your desire to help others. Perhaps you are trying to find the real answers to the questions of the life, therefore you become a volunteer to help those that have worse conditions of the life, than you have.
Dream dictionary: Volunteer dream meanings
If the one is dreaming that he is offering himself some volunteer work, in order to complete a difficult task or simply because of patriotism, then it is a clear indication and announcement that he will get recognition and satisfaction.
To dream that you are a volunteer indicates that you anticipate the future and will begin a new phase in your life. This dream my also show your determination to do something. To dream that you leave something voluntarily, represents that you should review past situations and learn from them.