Some guy: "I saw you training at my gym the other day. I was going to come say hi, but you looked pissed off."
Me: "I wasn't pissed off. I was TRAINING."
You're in the gym to train, not to socialize. Training requires a great degree of concentration. Your gym-face should reflect your attitude about your training. Don't wear your gym-face to the club, and don't wear your club-face to the gym.
I see a lot of people in the gym wearing their club-face, socializing, and having a great time. And if that's what you're there for, that's fine, as long as you stay out of the way. Not everyone wants to grow as much muscle mass as possible or get maximally strong.
But if you do, then you need to train like it and take your time in the gym seriously. People often lose focus because they have very mundane workout plans that don't actually require them to focus. In other words, there's no reason to concentrate or get your gym-face on because your training program doesn't require it. Or maybe you just suck at training really hard.
Back in the day, one of the biggest staples of leg training was high-rep work. Twenty-rep squats, 50-rep leg presses. You know, hard shit. Guys used set-extending techniques like drop sets, rest-pause, running the rack, and all sorts of methods that drew the set out as long as possible and created a ton of metabolic stress. They didn't even know what the term "metabolic stress" was. All they knew was that when they trained really, really hard, they grew.
Today we see the internet filled with memes like "anything over three reps is cardio." Look man, even the hardest of three-rep maximal sets can't be compared with a high-rep, all-out set of squats. It's not that a three-rep max isn't hard, but the two just aren't comparable. At some point in your training life, if you want to move forward you're going to have to train your ass off.
Two Intensity-Extending Techniques
Add in one of these intensity-extending techniques that forces you to set personal goals at each training session.
The working set of this technique gives you a "built in" goal. After your warm ups, take the set to complete failure. Rest 60 seconds and do a second set, trying to get at least half the number of reps you got on the first set. So if you did 315 x 10-12 on the first set, you'd need to shoot for at least 5-6 reps on the second set. This works great for pressing and pulling movements.
Goal-Focused Rest-Pause Sets
Do a set, rest 30 seconds, do a second set, rest 30 seconds, and then do a final set. If you got 100 pounds for 15 reps on your first set, 9 reps on your second set, and then 5 reps on your final set, that's 29 reps. The goal for the next week is to beat 29 total reps.
At some point there has to be more weight added to the bar or more reps done with the same or more weight. This doesn't mean you sacrifice form or movement execution. It simply means that progressive overload, especially for the small and the weak, should be the cornerstone of your training ideology.