Superman: But, when I save a life, in that instant, I know two things that most people will never figure out: Why I'm here, and how I can make a difference.
Martha: So now tell me more about this woman you're going to Lex Luthor's ball with.
Clark: Lois is... well she's complicated. Domineering, uncompromising, pig headed... brilliant.
Lois Lane: Danger is my business.
Jimmy: How could you see that from all the way over there?
Clark: I have a... really strong prescription.
Clark Kent: I just wanted to say goodbye.
Lois Lane: Goodbye? We're partners.
Clark Kent: You don't need a partner Lois, you never did.
Lois Lane: Well, maybe not, but I was starting to like having one.
Emmet Vale: We'll just put the word out on the street that we need a donor.
Rollie: Emmet, we're scientists, we don't know anybody "on the street."
Lois Lane: I win, you lose, we're both happy.
Perry: Yeah, well, just remember there's no perfect sunsets. There's a little crack in every cloud, but that's what gives you your silver lining.
Jonathan Kent: Lois, Clark is strong. And I'm not talking about how much he can bench press. He's strong where it counts, and when it counts.
Lois Lane: So you're saying you would never lie to your wife, that's assuming someone would actually be crazy enough to say "I do' to you?
Lois Lane: Clark, you can do the horizontal rhumba with the entire Met-Net cheerleading squad for all I care, just keep your hands off my copy.
Perry: Real nice kid, millionaire by the time he was your age.
Jimmy: Yeah, well I had the mumps in sixth grade, kind of slowed me down.
Lois Lane: You still think I'm crazy?
Clark Kent: I think you're brilliant. But there is a fine line between brilliance and lunacy.
Lois Lane: Well, I like my quirks. I think they make me unique.
Clark Kent: You certainly are unique.
Tempus: Oh please, I'll go to jail, I'll strap myself into the electric chair, just don't make me listen to this.
Lois Lane: What do I know? These glasses fooled me for two years. Oh boy, what a dope.
Lois Lane: You took advantage of our privileged interview session to steal highly incriminating evidence from an unsuspecting subject... Oh! I love that! Mmmm.
Lois Lane: Jimmy, give me back my dress.
Clark Kent: Now there's something you don't hear around the newsroom everyday.
Lois Lane: Well, that's a terrific reason to get married, to avoid harassment.
Answer: I don't recall if it was ever mentioned in an episode, but it's possible that the frames and/or lenses of his glasses are lined with lead, the one compound his X-ray vision can't penetrate. Speculative, of course, but it likely would have been done at a point in his youth when he wasn't in complete control of his powers.
Cubs Fan ★
I think the other answer has good speculation. I would add it could also just be an "acting gesture" that Dean Cain utilizes as part of his Superman character and not because there's any practical reason. That way, the audience immediately recognizes what he is doing.
raywest ★