Well there are certain positions that you feel most comfertable in, and those are the positions that you most likely get turned on in. I for one can't stand up and jack off, I can't seemed to get turned on enough. Sitting works best for me, but lying I can do too. Maybe it's all pyschological, I really never thought about it before.
Maybe because you are only comfortable lying down, or your body is used to doing it that way? Perhaps standing just takes too much effort, and distracts your body from what you are trying to concentrate on.
How come it seems like I can only get the rush from masturbating while laying down.
Because lying down is just how it works for you. I can myself only masturbate while lying down. Facedown for me to masturbate myself but I can still orgasm if a girl does it while lying face up. Its just how we work.