This can be either lifting weight, or doing cardio. They are different forms, but still have to deal with tolerance of hard work. Without that feeling of pain, or suffering, you won't get the results you expect. You need to get through that to get where you want to be. People tend to not understand the importance of pushing through that pain to get to the other side.
First, there are the people who just say it isn't worth it. If you say you want to change your life, but aren't willing to deal with the pain, you don't want to change your life. Pain in the gym to change your body isn't the only kind of pain. Psychological pain can be just as hard to deal with, if not harder. Changing your mentality, or lifestyle can be extremely tough to do. The bottom line is; if you want change, you have to learn to deal with it. It's the only obstacle between you and what you want, what you dream of.
The physical pain in the gym: When people say they can't do it anymore, they can't push through the last reps, it isn't because they can't, it's because they lack the will to continue. 90% of this is mental. When you can't push through those last reps, it's because you choose not to. That pain will not kill you; it'll only increase the benefits as soon as it subsides. If you put that into perspective as you try to push through them, it may help you.
Basically, if you can master the mental side of dealing with pain, you're good to go. If you're an extremely competitive person, it makes it much easier to do. You'll be much more willing to take the pain and beat the odds, or your competition because success is so important to you. It ultimately boils down to who is willing to overcome the most to get to the best spot. If you aren't even willing to pick up a dumbbell and try a bicep curl, you won't be anywhere near the big biceps you dream of no matter how much you know about training. The guy who walks into the gym with no knowledge, but the will and desire to do whatever it takes to succeed; he will be the one who gets what he wants with his body, and in life.