I'll make a list because it's simpler and I'm a little sleepy:
Reason #1: Everyone likes to have the content they submit viewed and appreciated by other fans. I, for once, like it a lot when people view what I've added, rate it and comment. Makes me feel like I've done my part in the Fanpop community and like I've pleased others (which I like);
Reason #2: I'm not exactly sure how it works but, from what I've read, rating plays a huge role in helping a pending spot become a full spot and for me that's important because not only I think that a lot of the spots deserve to be full spots. I have contributed quite a bit to a number of spots that are not official yet and I'm thinking part of it it's because of the lack of ratings;
Reason #3: Rating everything you see takes about 5 seconds, it doesn't hurt and it helps make fanpop better: it encourages new users and old users to contribute, shows users that the content they have submitted is good and, in case of bad ratings, what the user needs to improve in the future (credit, right placing, good tagging, etc);
Reason #4: Not the stronger reason but it looks pretty seeing everything rated and, therefore, green! (green is good, is echologist! =D) and it also looks pretty having the medal in your profile full of numbers under it...;
Reason #5: Rating shows you care about others, about content, about the spots you joined and about Fanpop in general because it shows you have recognize the work others have put into show everyone the things they are passionated about;
Reason #6: The medals. I confess, I like getting medals because it's a recognition of the work, time and effort I have put into spots about my favorite things. If everyone rates fairly and objectavely, people who truly deserve will get medals instead of those who just get them because they create fake accounts and such to get ratings (this is mentioned in a forum in this spot, that's why I'm mentioning it).
I think that's about it, no more wisdom (ha ha) to share. I hope you enjoy the article and, of course, feel free to comment either you agree, disagree or want to add something else.
Oh just in a little side note, don't rate because it's polite or because of your Fanpop reputation (lol). Rate because it's the right thing to do and it shows your appreciation. Also if you rate something low, leave a comment explaining why you did it so that others can improve. And don't be discouraged and give up if you see a lot of things unrated, rate anyway because even if others don't bother with that, you can still do your part!
