[xubuntu] Ubuntu Server 13.04: Won't install xubuntu-desktop.

Hi.

I swear I've been doing this for a long time, but this time was different.

I start with ubuntu-server, this time 13.04. Then when that's all set up nicely I install xubuntu-desktop (or whatever desktop.) I went into aptitude, selected xubuntu-desktop, and when I said go it said something like "can't find a reason to install xubuntu-desktop."

Huh?

Can't exactly tell you what the message was because I promptly exited out of aptitude and did it with apt-get install xubuntu-desktop, which worked fine.

Why would aptitude refuse to install xubuntu-desktop?