I am guessing that you pretty much ride the same ride the same way every time you ride. Nothing wrong with that if you are happy with your legs performing the same way every time you ride.
Your legs are lazy, they would just as soon be at home in bed. Once you get them on the bike they are thinking ,Ok we know this routine ,we''ll do it as long as you don't ask for anything extra.
They are lazy and they have no reservations about lieing to you either. I am sure you have heard this before. Oh we're so tired ,couldnt possibly pedal one bit faster. Oh shift down this is too steep for us.
The trick is to train them. If you ususally stop for a break at the top of your big hill your legs are thinking, All right we'll get your big ass up there as long as we get that nice break at the top. Next time get up and sprint the last 100 meters, explode over the top and keep going hard into the descent. Legs will be shocked, horrified , but probabaly performed just fine. After doing that a couple of times they are thinking,Ok we have to get his big ass up this hill and be ready for the sprint at the end.
Start messing with your legs sense of security. Dont stop where you always stop. Throw in jump sprints randomly. Pretty soon your legs wil be thinking, Ok got to be ready for anything , never know what this guy is going to want to do next.