Writing statements of purpose (SOP's) can be challenging. If you make the statement too general, it may come across like you don't have a solid reason for attending a graduate program. On the other hand, if your SOP is too specific, you may unnecessarily restrict your chances of admission—for instance, if you only talk about working with one or two professors, and they're not taking new students, you may not be admitted.
That said, you are free to write your statement of purpose however you choose. Just keep in mind that in formal application processes, you are generally writing to an entire department, not just one or two professors.