Very few people knew exactly why Myrtle spent her time crying in the toilets.
The Golden Trio, along with those who knew the circumstances of her death, assumed that she haunted the girls' bathroom because that was where she died, and that her violent and untimely death was responsible for her melancholy psyche. Some of the younger students thought maybe she was just a melodramatic, annoyingly angsty ghost who enjoyed making the toilets overflow and generally being a nuisance. Malfoy assumed it was because she liked sneaking into the prefect's bathroom to watch him bathe (after all, who wouldn't enjoy seeing a Malfoy in the buff?), and that she (naturally) wept copious tears at being unable to have him.
These assumptions were all true to varying degrees. Certainly, being killed by a giant basilisk while one was in the midst of a teary adolescent fit of temper tended to impress a rather pessimistic view of life upon the individual involved. At the same time, her personality prior to death had already been of a somewhat gloomy bent, so she did tend to go a bit overboard now that she was a ghost and could do things she couldn't before. Lastly, nobody could deny that Malfoy had a fine, fine body, and Myrtle certainly did regret the loss of her corporeality at certain times (namely when Malfoy rose from the tub in all his naked glory, little rivulets of water running down his chest and dripping off his...). But there was one other source for Moaning Myrtle's never-ending depression that nobody knew about.
Try as she might, Myrtle could never sneak into the Headmaster's bath.
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This may remain as a oneshot, or if anybody is interested, I might develop it further. Let me know what you think!