QUESTION: I am a little confused about the phrase "standing must be  jurisdictional," especially in view of Steel Co. v. Citizens for a Better Environment. In that case, the Supreme Court held that because  none of the relief sought would likely remedy respondent's alleged injury in  fact, respondent lacked standing to maintain this suit, and the courts lack  jurisdiction to entertain it. My interpretation of this holding is that  because the resdressability element of standing is not met, the court lacks  jurisdiction, but that doesn't seem right. Could you please elaborate on the  Steel Co. decision?
ANSWER: I think your interpretation of the holding in  Steel Co. is correct: the plaintiffs lacked standing because they failed to  establish that any of the forms of relief they sought would redress their  alleged injury in fact. This is, in fact, exactly what the Court holds.  Redressability is a part of the standing requirement, recall, because if the  court's decision will not give the plaintiff any meaningful relief from its  harm, then the opinion is essentially advisory. And the basic animating  principle in this area is that Article III prohibits federal courts from issuing  advisory opinions.
The first point you raise -- that standing is jurisdictional -- is distinct from the question why Citizens for a Better Environment lacked standing. It means that (1) the court must address standing first, and resolve that the parties have standing, before going on to address the merits, and (2) that the court must address the issue even if the parties have not raised it, for subject matter jurisdiction cannot be waived -- it is doctrine, rooted in the separation of powers, that constrains the federal courts.
The first point you raise -- that standing is jurisdictional -- is distinct from the question why Citizens for a Better Environment lacked standing. It means that (1) the court must address standing first, and resolve that the parties have standing, before going on to address the merits, and (2) that the court must address the issue even if the parties have not raised it, for subject matter jurisdiction cannot be waived -- it is doctrine, rooted in the separation of powers, that constrains the federal courts.
 QUESTION: So, in addition to the three  elements for standing: injury-in-fact, causation, and redressability, there  always needs to be a jurisdiction without which the courts can't speak. Is that  right?  
ANSWER: Yes, there always must be jurisdiction. Jurisdiction -- in its Latin roots  -- means the power to speak. So, no jurisdiction, no speaking (by the court).  Standing is one aspect of jurisdiction, but it is not the whole of it. The  dispute also has to present a question fitting within the subject matter  jurisdiction of Art. III, sec. 2 (e.g., a question of federal law), and it  cannot present a political question. 
ANSWER: The two ideas are nested. Redressability is part of the standing inquiry (but not  all of it), and standing is a part of what we consider jurisdictional (but not  all of it).