Sports require certain personality traits or physical attributes that will determine a persons level of success in that sport. Runners should be lean and have low-twitch muscle fibres. Boxers need good hand-eye co-ordination and a strong jaw. Tennis players need good co-ordination and stamina, and so on.
Looking back on 20 or so years of weight training and seeing different people come in and out of the gym over the years, the attribute that strikes me as most important for a body builder/weight trainer is persistence. Going the natural route, it takes years of hard work in the gym to build a body that is going to be BIG, where people will say to you, 'Dude, I think you may be too muscular'. If it were just a matter of time, that might be ok, but one needs to spend consistent time on the gym over a period of years. I've seen numerous good friends come and go from the gym over the years, wanting to put on serious muscle but getting disappointed when they're not Mr Olympia sized after 3 months of 'hard work' (lets ignore the steroid abusing pro body builders please :)).
Gaining muscle, strength and power, requires pushing the body beyond what it is comfortable doing. It doesn't get easier as you get stronger. If anything, it gets harder. As the weight increases so does the stress on joints and tendons, and the chances of injury. Breaking your muscles down to build them up also causes pain. To succeed at body building you need to be able to endure. Seems paradoxical that in an activity so focused around high-twitch muscles and short sharp bursts of power, endurance is so important. It's mental endurance that counts here, not so much physical endurance, although having a body that isn't going to break down and stop after 6 months is certainly necessary.
Remember what I said just above, about how it take years to build a decent body? That's years of near constant pain, pain that can make you not want to move at times. I'm of the opinion that if you don't hurt, you're not pushing hard enough.
The other thing about BB/WT is that it's a solo sport. There are no excuses if you don't reach your goals (well, besides the cruel hand you may be been dealt by genetics :)). No one but you is going to push you to go to the gym, or to go to bed to get enough rest. No one else is going to make sure you get the right diet, and combine all these things year in year out. Having a training partner helps, but ultimately they don't know just how far you can push an those last few reps.
Increasing in mass, strength, and power is about personal focus, you get to learn something important about yourself; you gain understanding of just how much you can endure and how much you can push beyond what's comfortable. It's a great happy feeling as that massive surge endorphins come out to play, when you really start to tax your body. It's like you can and will conquer everything. Like a 'runners high'. I know I've had a good session when I hit that. It makes me smile, even laugh in the gym at times ( I must look mighty strange to the random people bouncing along on the treadmills).