It's not hard.
...sorry. I had to.
Anyway, sometimes we do, sometimes we don't. It depends on the model. That said, there are a few factors that make it harder.
To get hard.
God, it's tough to do this without endlessly punning.
Anyway
The first thing is that your first shoot will be... harder... (sigh) than your fiftieth. I know, I know, it's hard (DAMN!) to believe, but eventually you just get used to tickling beautiful women and it's not the insanely exciting experience it is when it's something you never get to do. Secondly, half the time you're not even concentrating on the model in front of you; you're watching your monitor to make sure the shot is lined up and the lighting is correct and oh don't forget to flash the greycard for white balancing and all that jazz. Honestly, given everything you have to keep track of sometimes you even forget there's someone else in the room.
Finally, it's about making a product, not about your dick. Having a business mindset about the experience and concentrating on how to best make that product keeps you from focusing on how much fun you "should" be having. And, and I guess this is a personal thing, but I have a hard time (STAAAHHHHP) getting it up for someone who I know is just there for the job. If they were there for me specifically, I'd be way more into it.
I mean, don't get me wrong, when I work with someone I really click with and they turn out some great material then it's awesome, and sometimes I even come away from it thinking "damn, I'd love to see/work with her again", but that's basically it.