I like this topic because most people are too cool to and hardcore to talk about it. They claim they never lose motivation and if you do then you just don't want it bad enough lol. It happens to everyone at every level. For me there are varying degrees to it. When I first started bodybuilding I was delusional and thought if I worked hard enough I would become Mr. Olympia lol. So after I realized that wasn't in the cards no matter how far I took it, it was tough to be as motivated as I was before. Ignorance is bliss I guess. Well that was some time ago and needless to say I don't feel that way as far as not being motivated, at least not normally.
Motivation for me comes and goes though, sometimes I want to rip heads off in the gym and other times I have to fight through my workout and keep my goals in mind even when I don't really care that particular day. Dieting is the same way. I've learned I'll come through those times where motivation is low and be better than I was before, so I never go off track as hard as it may be some days. For me I just question the point of all of it sometimes, the pain my body is in all the time, planning 99% of my meals every day, etc. However the reality is I enjoy it overall, and nothing feels better than when I see how much I've changed since last year or the year before. Doesn't matter if its to compete or go to the beach, set your own goals and find your reasons for accomplishing them and go ahead and do the shit you need to do every day, because the motivation will return at some point, so why not progress in the mean time.
My take anyway.