Why do we live on earth? Why can't we live on other planets??

The first part
of this question has been thoroughly answered. However, the second part comes from an incorrect
premise.

We can live on other planets. It's just that we
need a very large amount of technology and supplies in order to do so. Currently, the cost of
this support is too large for us to feasibly establish a colony on another world, and there are
no locations we have yet discovered which do not require extensive life support just to get
there, let alone to live there.

Terraforming is one procedure that might
circumvent this. Terraforming is the act of altering a planet's ecosystem in order to be more
like Earth. Some extreme examples imagine turning Mars or Venus into "second Earths"
complete with oceans and atmospheres, but this is a bit of a stretch. Mars lacks important
features, such as an active magnetic field, that would be necessary for habitability and cannot
be engineered through current human technology.

However, terraforming could
create localized regions in which human life is, at least, more generously supported than the
rest of the planet. For example, living underground on Mars would be a lot easier than living on
the surface, or trying to alter the entire planet.