Here's what I've noticed about what I do when I see somebody doing something that requires skill, and I want to do it myself.

First time:

I do it quite well the first time.

• Saw how the other person was doing it
• Not thinking about making a mistake
• Just going with my instincts

Second time:

I do it quite badly the second time.

• Too confident, since it was so easy the first time
• Not being as cautious or rational about it
• Have totally forgotten about what I was doing right (which I learned from the experienced person)

Third time:

I do it even worse the third time.

• I'm anxious and confused now because it was so easy the first time, and it suddenly it became much more difficult the second time
• I'll be paying more attention to the mistakes I made the first and second times than how the experienced person was doing it
• Not relaxed

And that's why thinking is bad.