I needed to learn this.
In part it took time because I needed to develop confidence in my body. I needed to learn about it. What would happen to it as the weights got heavy and as it failed. I remember being really scared of pushing things hard on the shoulder press (machine version) because I was afraid that all of a sudden my muscles would just give out and the weight would come crashing down on top of me. It took some time of playing around before I got to learn that my grip won't give out first (or my arms won't raise what my grip can't secure) and that failure involves the muscles not being able to make the weight move up anymore - but they are still good to static hold it for a few seconds and they are still good to lower it slowly and safely.
With pushing myself... What really worked for me was having a program where I recorded what I did each and every time I trained.
I was supposed to start (according to my program) with a weight I could lift for 3 sets of 10 reps. It took me several sessions to figure out what weight was suitable for that. I adapted it to 8-12 reps. If I couldn't do 8 on the last set then the weight was too heavy. If I could do 10 thinking I had another rep left / if I could do 12 reps then the weight went up for all sets next time.
Putting the weight up was important to me. It meant progress. It meant I got stronger / could do more than I could do before. When I moved to free weights it took me a while to get that same sense of satisfaction (which drove me to bloody well try and get it so the weight could go up). Mostly because I worried about my form. Seemed that minor differences in the way I held myself made things easier or harder and I felt like I wasn't sure whether I was doing it properly (so deserved the increase) or whether I was cheating (so did not).
I'd tell him that the momentary pain does kinda suck.... But that it is followed by one of the best feelings in the world once you have either accomplished something really worthwhile and / or once you get to say 'I really gave that my all!'
I really do think that people can learn this in time. It does take some time, though. Having good role models is most important, I think.
It really does surprise me how many people aren't interested in increasing the weight. I've had a few friends express great surprise that training (for me) was about getting to put the weight up! They didn't seem to realize that the body adapts so then you gotta ask more of it which gets it to adapt and so on... And that that is what progress is about.
I do think that a lot of people who go to the gym really do just think that they want to maintain things. They don't want to see their bodies slide into decrepitude... But they don't seem to understand that they are capable (if they work hard) of being so very much stronger and better looking than they are at present!
I didn't go to the gym for the longest time because I thought it was a place where all the fit and healthy people stood around admiring themselves and flirting with each other. Most of the people do indeed seem to be doing that... But there are a few who get that it is about individual progress. Learning about that has been totally life changing for me. I didn't know I had it in me. Maybe he doesn't either.