For the most part, I agree with what xnine said. I started writing fan-fiction because I wasn't particularly a creative individual, so it was very easy to take an already existing world and write about it, or add in my own character. I don't write fan-fiction anymore, but it wasn't that long ago that I did.
There is absolutely nothing inherently wrong with writing fan-fiction. So many people already do it and they are not inherently weird because they do. You may find a lot of fan-fiction you don't like for many reasons. Most of these reasons are that the writer is inexperienced, juvenile, cliché, inaccurate, or displays the characters in out-of-character (OOC) contexts. It's difficult to properly portray an existing character in the way they would logically follow; that requires a thorough understanding of who they are. On the contrary, it take a lot of psychology and understanding of getting into that character's mind. Writing appropriate fan-fiction is not easy.
If you are hurt because your friends laughed that you wrote some, well there are likely a variety of reasons for that. I don't know if they're friends in real life or on-line, but people will find a reason to laugh at something they find weird or different, especially if they're between middle-school and high-school ages. I knew people growing up who were into anime in general, and although they had friends who were like-minded, they were generally shunned because of it. At a time when wanting to be cool is the norm and childish things aren't, liking cartoons—which people basically group anime into—becomes childish. People generally tend to grow out of this in late high-school and college
(Exhibit A). A lot of people will retain their disgust for it, though. If you ask your friends why it's so weird or why they don't like it, I assure you that their answer will most likely be unknown, "I don't know, it just is," or simplistic, "because it's lame/stupid." People generally have no response beyond laughter if you get offended for it, so there's no helping you but either keeping it to yourself or maintaining confidence in your stance. I wish you luck in that regard.
Back on-topic, writing fan-fiction is a great way to start writing altogether. It can prepare you for writing your own original stories. Although it is my personal belief that it should be used as a stepping-stone in order to get you writing original tales, there is no necessary reason to stop if you just enjoy it that much or lack enough creativity to make it happen, though you should strive to. Some authors think of fan-fiction as the worst thing gifted to the Earth since the Plague
(Exhibit B). Factually speaking—however—some of the greatest classics in history are fan-fiction
(Exhibit C). The thing you should remember is that you are empowered by your own drive. If you succumb your efforts to submit to those around you, your drive will die off and you will conform. If you enjoy writing fan-fiction, then you must continue to write it for the sake of creating, which the practice of is a blessing on its own. If you grow out of it, then that's fine, as long as it's your decision. No one here will insult you just for writing fan-fiction (at least, not with me around to say something about it), and quite honestly a lot of people would like to see it, I'm sure. So, as xnine said, feel free.
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