Your statement of purpose is something like an academic job interview, or at least that is one way to think about it. Your SOP should sell you to the university. Remember you are vying with others not only for admissions, but also important funding. Therefore, it is necessary to present yourself in the best light possible. It is also important to clearly illustrate why you are choosing this particular program and how it fits in with your larger life goals.
For an admissions essay, it should not dwell on how your interests developed, but it should outline your qualifications, the contributions you could make to the university's program, and especially areas of possible research you would like to pursue. A little background on how you “discovered” your field of study is nice, but that doesn't pack the same punch as illustrating how you've pursued and contributed to it since you “found it.”
Cite particulars about the university that attracted you to it in particular; admissions officers are not likely to be moved by the idea you chose the university because of its well-organized Web site or you've always been a big fan of the basketball team. It would be better to mention particular programs or research initiatives or even particular members of the teaching faculty whose research you are familiar with that attracted you to this university. Statements like you "want to help mankind" is unlikely to differentiate you from other applicants and set you apart. Give reasons for choosing this university over another. What graduate (and undergraduate) admissions counselors are looking for is someone with clear, definable goals and a good reason to be seeking admission into this particular university. Then they are much more likely to feel the student is a "good fit" and consider them for admission.
A particularly nice site for this is www.statementofpurpose.com. Be careful about this site though--it is a front for an essay review service, but it still provides some helpful information to get you started.