You can even spice it up by doing it from the sides and please your guy more. I really can't think of any time I've gone against that "mantra". Some girls also feel uncomfortable being on top because they feel exposed, pressured, or self conscious. Besides, if you position yourself well, you will be able to ride his dick for a long time before you get tired. Then slowly use your hand to guide his hard penis into your vagina.
Same problem as Lonnie.
How To Ride A Man: 15 Tips On How To Give Him The Best Sex Ever
Then once you are comfortable with that, try moving it up a notch using an easy to perform position like the Man Missionary. One thing nobody mentioned so far is make sure you position yourself on him so that he is comfortable. Same problem as Lonnie. However, some guys prefer to look at your hair tied on the back. Does it make the girl unattractive? Maybe she just feels a little daunted by the prospect of doing all the work, so let her know you're there to help in simple ways like that. This can make him slip out very frequently.