For me it's a combination of someone I like, compelling title/preview, and how much time I have. Sometimes even my most favorite deviants make boring journals or journals that don't have anything to do with me (business, etc.). For groups, I ALWAYS have journals turned on even though deviations are off so I can keep track of contests and changes.
If it's a journal that I REALLY REALLY wanna read but don't have the time, then I'll just leave it until I do have time. o;
Journals are too long for my interests. The Internet has changed my behavior on consuming content, and walls of text aren't something I want to view. I usually close them all out unless it's something that sounds really compelling, then I'll skim it. But mostly, they're TL;DR.