Go hard or go home right? Isn't that what they say? Lift as heavy as you can every session or you are weak and lazy right? If you don't push it to the point where you feel like passing out you are not training hard enough right? WRONG!
When I started working as a Personal trainer that was my attitude and the attitude of my colleagues and mentors. If myself or a client didn't push it HARD every single session I would put it down to just laziness. I learned a very important lesson though no-one is that invincible. We are not machines and on any given day your body could be dealing with various things that could affect your energy levels and the level of effort at which you can apply to a training session. It's also not in the average person (working full time, kids, friends, little regular sleep) best interest to smash themselves like that every session if at all. Your body will not recover from that kind of stress.
I do think there is a lot of misconception out there about exercising and what it's really all about. Its meant to make you feel energised, invigorated and challenged not sick. You need to train based on how your energy levels are on a given day. Now I'm not saying don't train hard in anyway what I'm saying is train hard on the days when your energy will allow it and tone it down on the days where you are stressed, exhausted or really really sore. Let me repeat myself this is not a free pass for the lazy people to take it easy. Training to the point of exhaustion on the low days will only cause your body more harm than good and it's just stupid.
Unless you are an athlete (who has time to sleep during the day to recover) training consistently is what will get you the result, not training to the point of vomiting every time and then burning out a few months down the track. Remember what I said about stress and how it effects your weight loss goals? Read here
Understand that training is indeed a stress in itself. When you do a resistance/weights workout you are actually causing minor micro tears in your muscle fibres. When you get adequate rest and refuel with protein your body is able to repair and grow even stronger. If you are unable to get adequate rest and you are not re-fueling your body properly it is hard for your body to fully repair. So retraining legs when you can hardly walk from DOMS (delayed onset muscle soreness) is not in your best interest and you will not get the gains you want anyway. If anything you will put yourself in a one-down position in terms of your results.
Train smart. Train based on your circumstances. Ensure you have adequate rest. Refuel your body with vital nutrition to help it repair. Be consistent. Take the pressure off. Do all this and you will be successful.