"...it bothers me that we're not mentally tougher..."If you follow the Celtics like I do, you'll know that they've lost 3 out of the last 4 games (Clippers, Sixers, and Nets). These are games you would think that the #1 seeded team in the East could win quite easily.
One could chalk it up to injuries, fatigue, back-to-back games, new players, or whatever other reason that's been talked about on the Celtics blogs and forums, which I read on a daily basis. However, Doc hit it right on the nose when he said that his team is just not mentally tougher. The mental toughness and thoughts of players set the tone for how they play in the game.
Why do coaches give "pep" talks before a game? Why do players have certain rituals before a game? They're "psyching" themselves up to be mentally prepared.
Why do players like to trash-talk their opponents? They want to get inside their head and affect how they play.
How are teams able to come back and win a game after being down 20+ points? They believe and have the confidence in their game and abilities.
How can someone who is hurt (Kobe) or sick (remember Michael Jordan?) continue to play at a high level? They stay focused on their game instead of their physical ailment.
Believing and knowing that you can win far outweighs any physical skill that another team has. Look at the Celtics. They have three future Hall-of-Fame candidates who are extremely talented in their position. They also have an assist master in Rajon Rondo...and yet, they were beaten by teams that are probably not even going to make it to the playoffs. If this were the playoffs, do you really think that the Celtics would have let this happen? I don't think so. That's because when you're in the playoffs the whole mindset changes.
Here's an article written by Dr. Alan Goldberg that goes further into sports psychology and mental toughness.