Click here to see detailed information for every writing trait from the Northwest Regional Education Laboratory.
Do you care about your topic? Do you care about your reader? This and much more is conveyed through your writing. Your attitude, like your dress, needs to be appropriate to the situation. Would you wear a bathing suit to a college admissions interview? No. It wouldn't be appropriate.
In a similar manner, an informal voice or tone in the context of a thesis-based research paper for a sociology class wouldn't be a good choice. In a reflection paper for your English class, however, the informal "I" voice is what is most appropriate.
"Should I Use 'I'?"- a great resource from the University of North Carolina, Chapel Hill.
"Tone: A Matter of Attitude"- an interesting and informative page from The Guide to Grammar and Writing.