Paul D’Amour speaks in the latest issue of Bass Player magazine, mostly discussing his work with Lesser Key, however he does touch on his time with Tool and his reasons for leaving:

How did your time in Tool affect you as a musician?
That experience changed my DNA. To go from 0 to 100 miles an hour in the span of a year was incredible. All of a sudden, we were playing in front of huge crowds, and we had a ton of success. It gave me a lot of confidence to do anything I wanted to.

Do you have any regrets about leaving Tool?
I wish it had been a better vehicle for me to create in, but it just wasn’t. Their creative process is excruciating and tedious, and I guess I never felt the desire to play a riff 500 times before I can confirm that it’s good; that’s why it takes them eight years to write an album. I always wanted to do other things, and it felt like I was too much in a box with that band. They’re set up where the bass player does the bass part and the guitar player does the guitar part and so on. I couldn’t be stuck in that paradigm—it’s too stifling. I’m not just a bass player; I’m a creator, I wanted to have a bigger role, and it just wasn’t happening in that situation. In the end, I knew leaving was the right decision.

Go and buy a copy if you want to read the whole thing! Thanks to coreyisonreddit for the tip!