This. Fantasy is fantasy, and so long as you know the difference between imagination and reality, spinning yourself a hot little fantasy about doing a fictional character isn't at all equivalent to the whackos who think they're actually married to Snape on the astral plane and so forth. Even incorporating selected elements of that fantasy into a creative work isn't intrinsically a problem, if it's done with careful thought. But just putting your totally unfiltered private self-insert fantasy up as a piece of fanfic is the sort of oversharing that really doesn't tend to make a good impression on people.