Pushing > Turtling > Ganking > Pushing
If you want thwart push attempts, go gank-oriented.
As for why competitive teams favour early victories:
Dendi: “Okay, I’ll try to explain. People think that it’s pushing, but that is not really what it is. If you get tower, you get advantage, and I think the right way to play DotA is to pick up towers. If you can pick up towers, you get advantage. So if you take heroes that let you push six towers, you still have lategame, and you defend your six towers, you always win, because you have gold advantage. Lategame is very random. When you go lategame, you can lose any moment. Any mistake - small mistake. What is the point of playing lategame, if you can finish them in twenty minutes? You risk. Sixty minutes and boom, you lose the game. I think it is really not good at the moment.”
In short, given that both teams have a similar hero lineup, the longer you drag the time, the bigger the risks of losing since game time "amplifies" mistakes as heroes are more farmed up as time goes by (e.g. lose a teamfight in 15:00, and you might just lose a tower, whereas if you lose a teamfight in 50:00, you could instantly lose the game). It's only natural that players would want to secure their wins as early as possible.
By the way, 6 towers give around 9k gold to the team, so if you're able to defend your 6 while destroying the enemy's 6, your team would be sitting on a 9k gold advantage which can happen as early as 15 minutes into the game - no carries could farm that much alone by then. Not to mention the map control that comes with it (esp. that they wouldn't be able to farm NCs easily anymore, hence further reducing their gold income).
Last but not least, for those who want to improve their knowledge on this topic, read: http://www.playdota.com/guides/draft...r-intermediate