Many people are disappointed in Rocky, and this nameless coach echoes Mickey's sentiments later in the movie. One of the biggest reasons they're disappointed in him is that he's stubborn. He won't take advice.
ROCKY: Last year, me and the turtles had Spam.
Rocky doesn't have much of a personal life. He's very lonely, and without a family, it's just him, his turtles, and a can of potted meat on Thanksgiving.
ROCKY: Hey, you gotta be a moron, you gotta be a moron to wanna be a fighter, you know what I mean? It's a racket where you're almost guaranteed to end up a bum.
It seems like boxing is a last-resort career route, in Rocky's opinion. You don't get into boxing if you have other opportunities.
MICKEY: Cos you had the talent to become a good fighter! And instead of that, you became a leg-breaker to some cheap, second-rate loan shark! […] It's a waste of life!
Mickey is disappointed in Rocky for not putting 100% into his fighting career. He has never been able to forgive him, because Mickey always thought Rocky could be the best. This is Mickey's tough love.
ROCKY: I'm really a ham and egger. This guy, he's the best. It wouldn't be such a good fight.
Rocky doesn't think he even belongs in the same ring as Apollo Creed. After the verbal beating from Mickey, who suggests he retire, he's disappointed in himself, and has no confidence.
ROCKY: That's nice. You didn't take very good care of the picture, though.
This is a small, subtle line, but it shows Rocky's disappointment in how Mickey has let himself go. Sure, Mickey had a good career as a fighter, but he didn't take care of himself. Rocky knows if he had the same opportunity, he'd have cherished it.
ROCKY: Talkin' about your prime. What about my prime, Mick? At least you had a prime. I ain't had no prime! I ain't had nothin'!
This is one of the most emotional, heartbreaking lines in the movie. Rocky lets his frustration and disappointment with his own sad life bubble to the surface.
PAULIE: I don't get married because of you! You can't live by yourself! I put you two together, and you... don't you forget it! You owe me! You owe me!
Rocky isn't the only one disappointed with the cards life as dealt him. Paulie isn't fairing any better. He takes it out on Adrian, though, instead of taking responsibility for his own shortcomings.
PAULIE: I can't haul meat no more!
Okay, this line isn't nearly as good as Rocky's "prime" line. Paulie is hardly a prime cut of meat. But the emotions behind it are the same. Paulie is at his breaking point, and he's tired of begging Rocky to cut him a break.
ROCKY: That don't matter. Because I was nobody before.
The one positive thing to being at rock bottom is that you can only go up from there. As long as Rocky tries to fight Apollo, he will be better off.