How else does one install programs other than manually? You can't magically wish for Chrome to appear on ubuntu, you have to either use Synaptic, apt-get, or the software center (on recent ubuntii). All of these at least prompt you about dependencies.
'Manually' probably means not using a package manager - either with the configure/make dance, or with a binary installer like Google Earth used for a long time. In those cases, you still need to sort out dependencies manually.
Absolutely; if the dependencies exist within the appropriate Ubuntu repositories, or the ppas you have active, then the needed libraries can be installed.
Random downloading of debs from Web sites could lead to a situation where a different version of a dependency is needed. "foo needs libnaff-ubuntu345 but libnaff-ubuntu345 is not going to be installed" type errors result.
Ubuntu Software Center doesn't prompt about dependencies... it just goes ahead and installs them. At least that's the default behaviour on 12.04 on my two machines.
