Eh...It happens on occasion when you've been with the same person for a long time. Iv'e been with my wife for almost 15 yrs now. She's still smoking hot so that helps and the fact that she's a freak in the sack.....
One thing I might suggest is trying to hit a line drive up the 2 hole. That's exciting when she's not expecting it.
Try introducing things like toys, lingerie, posting nude pics on C.P. etc. That'll always help.