In general, when should one sue?

It's no surprise that "lawyer up" is tenet advice on reddit, but it's nothing something that's necessarily intuitively clear. Lawyers and legal experts, if you would take this opportunity to help anybody who comes across this thread, what general guidelines or circumstances would you suggest someone seeking counsel for? Additionally, which classical cases would you consider uncertain (or largely contingent upon the circumstances) and which cases would you say are almost always successful?

Edit: sorry, what I'm meaning to say is, "in which cases would one stand to have a good case for a lawsuit, but not be immediately aware of it"?