I thought Vegas linesmakers arrived at their spread in order to maintain that money placed on both teams remains as even as possible.
+1...I type s---l---o---w---l---y 
We can continue to get in a finite chicken or the egg argument. If it makes you feel cooler we can phrase it that Vegas arrives at these lines after careful evaluation of previous betting results and game outcomes to get the most equal amount of money on each side. However, this also means they are trying to predict how professionals gamblers (also sometimes referred to as sharps) will impact the line because they have much larger bankrolls and make much larger bets on individuals games where they feel they have an advantage.
A thing to keep in mind is that the average joe betting on a basketball game is betting a smaller amount $20 to $100 dollars being the most common amount for a regular season game. These average Joes, (or squares or fish, as they are commonly referred) represent a big % of the bets placed. In fact, betting sites will commonly try to mislead their members to the weaker side by putting out statistics like "80% of the bets have been on the celtics to cover the spread." However, if there are 300 $100 betters their impact on the line is offset by a single sharp placing a $30,000 wager.
These people placing the larger bets day in and day out are much smarter than you, I or John Hollinger. They use very sophisticated statistical models to predict games because their livelihood depends on. Unless someone has a huge income stream from other sources (see Floyd Merriweather), they will need to hit 55%-60% of their bets over the long term to swing a profit based on the amount of juice they are laying. If these people, the sharps that have the ability to move lines with their large bankrolls, are showing increased confidence in the Celtics that holds a lot of value with me.