The issue with that sentence is not that it's too casual, but that it's devoid of relevant content. Personal statements (like all academic writing) should be as factual as possible, and the facts should help the committee determine whether or not you would be a good candidate. That is an academic decision (are you strong enough and independent enough, are your interests a good fit for the department, e.g. is there a suitable advisor, etc.), not an emotional one (whether or not you would be a good buddy to hang out with in the evenings is, ideally, not part of the considerations). The people reading these statements are busy, they typically have to read a number of these, and teach, do research, and discharge other administrative duties the rest of the time, so you will help them by keeping your letter to the point. Waffle, whether in research papers or in personal statements, is annoying because it wastes time.
Note that there is no contradiction between being factual and being personal: statements about your academic and other related experience (e.g. relevant jobs and work placements, olympiads, etc), about the courses you have taken, about the courses you found most interesting, about the possible directions that you think you might want to take your research, and so on are relevant; dramatic descriptions of your emotions at a particular point in time are not.