You might want to ask yourself: "Do I really want to auto-update applications, which the applications that I build myself depend upon?"

Brew team [turned down this feature request to facilitate early updates][1]. 

My interpretation of their reasoning: "The risk of malicious code slipping through the opensource auditing process weighs more than the incidental need for a speedy update."

**If** this is your line of reasoning, you like to be in control of software updates, and you care a lot about homebrew packages being up-to-date, you are probably better off adding a line to the bottom of `.bashrc` (or the rc file of your favorite shell) *asking*:
```[bash]
brew outdated
```
This will confront you, in every new terminal, with the outdated packages. 
When your annoyance reaches the limit (you feel ready to update), you'll be happy to do the necessary.

  [1]: https://github.com/Homebrew/brew/issues/6382