Ask HN: Why do you use framework X?

After reading posts like http://news.ycombinator.com/item?id=712975 and http://news.ycombinator.com/item?id=750142, I find myself facing a decision regarding what technology to build a new project on. Rather than simply understanding numbers on how many of you use framework X, I'm far more interested in why you use it. Questions such as:

* What was your decision-making process when evaluating frameworks?

* Was your ultimate choice made simply because the framework was the defacto in your language of choice? In other words, did you choose Rails for Ruby, Django for Python, Cake for PHP, just because the language itself was what you knew?

* Assuming you're sufficiently experienced in framework X now, what are your regrets? What makes the other frameworks look like greener grass?

* If you could start your project over again, would you chose the framework again?

* Did you choose framework X secretly because you just wanted to learn it? In other words, was it attractive to you just because all the cool kids were doing it and you wanted to add it to your toolbox? Be honest.

Thanks!