Journal: Perpetual Sessions?

Recently I went to youtube.com from my secondary computer and realized I couldn't auto-login to the site. After logging in a few times across different computers I realized I could only be logged in on one browser at a time. But why? Is my security compromised by having a (hopefully) secure cookie across many browsers? Are there data issues? On my personal website, I implemented perpetual sessions with a few lines of php code. However many popular websites require you to log in for every session. Are perpetual sessions secure enough for everyday use on popular websites?