How to automatically fetch missing dependencies when installing software from .deb?

You can install a package and get dependencies from repositories with

sudo gdebi package.deb

If you already installed the package with missed dependencies, you can dowload and install dependencies automatically with

sudo apt-get -f install

Also available is a graphical version gdebi-gtk, linked to .deb nautilus right click action "Open With GDebi Package Installer".

From the 1.1 branch onwards, apt-get supports installing local packages along with dependencies in the way of:

sudo apt-get install ./your-package.deb

Note the ./ in front of package file name, which is mandatory otherwise the name will be used as package name, not a file name.

dpkg itself is not capable of managing repositories. A higher-level tool like apt-get is required to fetch anything from repositories. dkpg is only the core tool that installs/removes/configures packages, taking care of dependencies and other factors. apt-get and aptitude are tools that manage repositories, download data from them, and use dkpg to install/remove packages from them. This means that apt-get and aptitude can resolve dependencies and get required packages from repository, but dpkg cannot, because it knows nothing about repositories.