What is apt-get equivalent of "aptitude why"?

In this answer, I came across this command:

aptitude why firefox

aptitude whylooks like an useful command but I don't want to install aptitude just for that. What is the apt-get equivalent of aptitude why?

Instead of apt-get I would have a look at apt-cache, though not one on one the same but close will be:

apt-cache --installed rdepends firefox

People might want to know why an automatically installed package is present on the system. To get this information from the command line, you can use aptitude why package

apt-get has no similar feature

source:debian-handbook