A good player will change playing style's throughout a game simply because you don't always have the winning hand. It is like the song "You got to know when to hold-em, Know when to fold-em" (The Gambler).
You have to adjust to other players being seated at your table, in a large tourney, and you also need to slow down when you are moved to another table, so you can get some information on all the players already seated.
If you play too aggressive on a new table or against a new player you can soon lose a lot of chips, if he or they decide to be more aggressive player's,
The skill comes from practice, and the perception of the other players around you. Knowing when to slow down, or step on the gas is all dependent on position and chip power.
Poker is likened to chess, and in a way it is, because you always have to think about what you are going to do on the River even before the Flop is dealt. Being three steps ahead of your competition. is key to winning in both games.
When the fun Stops, STOP!