I hear you mate, I really do and I've accepted my views are simplistic at best and no doubt wrong at worst.
That said, in life you have to push yourself and football is no different. Take Sturridge, a player who his own manager has come out and said he needs to earn to deal with a little pain, to play through it, to just get on with doing what he does and that's play football. Its a physical game, a contact sport and you get kicked and you take the knocks and sometimes, its impossible to carry on and play but for me, the majority of the times its a case of mentally being strong and saying I'm going to do this, I'm going to play and do my job.
The poster above named several sporting events, all gruelling and as much a test of the mind and mentality as they are fitness and those competing, do it with niggles, aches and pains but they do it.
Seems to me that the modern day footballer wants to be different and wants to cry off at any and every opportunity and unless !00%, doesn't often want to know and that's sad. given the money and the adulation they receive.
Each individual is different in that regard and you have a good point in that players will have to play through injuries time and again and play at the level required.
Some have a better pain threshold than others.
Some get the flu for example and they are cactus and the club worries about its transmission to others for example.
Some feel crap and don't let on to others how bad they are and still play a pearler.
Its a small and perhaps trivial example but when I was little I really wanted to play first team football ( AFL variety ) for the school first eleven in year 10 which was unusual at the time and played a pearler in the practice match kicking 6 goals from a half forward flank when I was knackered with the flu and the rest is history.
Its how bad you want to play and how bad you want to win that often makes the difference.