You cannot shuffle whenever you want. As other answers have already pointed out, rule 401.2 says that you are not allowed to reorder your library.
I'll need this definition later, so I'll quote it now. The rule defining shuffling is 701.16a:
To shuffle a library or a face-down pile of cards, randomize the cards within it so that no player knows their order.
Even if you could shuffle whenever you want, it generally wouldn't actually help. To be specific, say you have 40 cards left in your library, and you just shuffled (perhaps because you just activated Evolving Wilds). Now, you know what cards are in your library, but by definition you don't know what order they're in. Then you draw a Forest, which you didn't want because you already have enough land. Now you still know what cards are in your library, because you can just subtract one Forest from the cards you knew were in there before. And you still don't know what order the cards are in. You know there was a Forest on top, but that doesn't help, because that Forest isn't in your library anymore.
Now imagine that you could shuffle your library right now. Obviously, the library now has the same cards as before. And again, by the definition of shuffling, you don't know what order the cards are in. And you should be able to see that this hasn't actually changed anything: both before and after shuffling, you know what cards are in the library, and both before and after shuffling, you don't know what order they're in.
Now, you might ask "But, if it doesn't change anything, why can't I shuffle whenever I want?" The answer is that I was describing the general case, and there are some cases where it does matter because you do know where some of the cards are. And then the ability to choose to shuffle would give you and advantage. [Jace, the Mind Sculptor]'s first ability would be completely useless if you could shuffle; your opponent could say "I leave that card on top" and then you could just say "Actually I'll just shuffle it away". Brainstorm would be much better if you could just draw 3 cards and then shuffle your 2 worst cards back into your library, instead of being forced to draw them back for the next two turns. Garruk's Horde would be crazy powerful: you could just shuffle until you see the creature you want, and then cast it.