I believe that the best way to begin a personal statement is to immediately begin with an interesting point about yourself that relates to the degree you're applying for. Beginning with "I want to study x because..." isn't a particularly engaging and original opening. It's best to try and open in a way that will interest the reader and show a real enthusiasm for the subject you are applying to study for. It is also important to remember to make sure the opening of your personal statement is relevant to the course you are applying for, you only have a limited amount of words to make a university want you as a student, so make every word count.