First, the question: Why train? Or even: Why workout? To stay healthy. To lose weight. To build self-confidence. To compete. To get stronger, run longer, look better etc. There are a lot of answers to that question and I'm not going say that any are wrong. Any reason to get somebody moving, to motivate them to care about their health and well-being is valuable. I know there are a couple of reasons on that short list that I train for. However, I'd like to propose that, above all, there is one reason we need to be training/working out - To live as full a life as possible for as long as possible.
Think about it. All the other reasons we may have for making fitness a priority really should come secondary to this one goal. Almost all other (healthy) reasons for wanting to be fit will follow this goal. If you train appropriately, so that you can live the long and full life you deserve, you'll find that you're more athletic, leaner, and you will feel better! If you make this one goal number one everything else will eventually fall into place.
Now, notice the use of the word eventually in there. This is important. Patience is key when you make a life of health and fitness your number one priority. If you're too busy focusing on immediate goals, you'll miss out on the chance to create long term, sustainable progress that you can carry with you for the rest of your life. Focus on living as full a life as possible for as long as possible and all the other goals you have will follow.
Again, I really want to emphasize that, for the most part, almost any reason that gets a person moving toward a healthier lifestyle is a reason worth having. But I also believe that placing this particular goal in front of all the rest will yield the greatest result for the rest of your life.
As far as 'Why Train?' I think it's the best reason you can focus on. That being said living as full a life as possible for as long as possible is not a goal that is only restricted to training/working out. In fact, this idea of a life of health and well-being can (and should) help inform you on the path of your daily life. This is where "The Spectrum of Movement" is going to come into play.
I like to imagine the things I do for fun, work, and training on a huge spectrum. Work is smack in the middle, it's the physical things I have to do. It might be running up and down the stairs at Wild Oats or demonstrating moves here at the studio. Regardless, it's movement and I need to be able to do it well. Now, on the training end of the spectrum I've got this - 2-3 KB Training/Week - Hill Sprints Twice/Week - Jogging 3-5 Miles/Week. So, here's where it gets a little tricky. To live the fullest life for the longest time, I need to train. Strength, Conditioning, and Auxiliary work are all important. However, to really live a full life, I also need to have fun. I'm not saying training can't be fun, but on the spectrum of movement it's at one end and fun physicality is at another. I train so that I can do what's fun, the things I love. I believe, that in order to really live a full life, the end of the spectrum we need to focus on are the things we do just to have fun.
I think there is a trap people are starting to fall into. Our lives are becoming more and more sedentary. So, a natural response to this problem is to get moving! However, it seems these days that 'Get Moving!' is trending more and more towards 'Get Training!' or 'Get Working-Out!' This is not necessarily a bad thing, but I think something huge is getting lost here...and that's movement for the pure joy of it. Humans were meant to move; we move to survive, but we also move to have fun. I think the spectrum of movement is shrinking and as it happens we are losing a huge chunk of what is vital to a truly full life. I love to train, but I do not want to spend hours in a gym. I want to be riding my bike, trail running, backpacking, slack-lining, swimming, mountain biking, playing frisbee (of both the ultimate and golf varieties). I love doing these things! I don't do them because I think I need to, I do them because I want to. This is a huge element to a full life and to having a wide and healthy 'Spectrum of Movement' that is getting lost for many people. If your only chance to be physical, outside of work, comes in the form of training or only happens at a gym you're missing a huge opportunity to add another element of fullness to your life.
Train to stay healthy, but stay healthy so you can do the things you love. All of those fun activities I mentioned above (and more!) I have designs on doing for the rest of my life. I've only gotten more enthusiastic about them, only found them more essential, as I've gotten further along on my own fitness journey. I'm not saying those need to be the same things everyone does to find the joy of movement in their lives, I'm just suggesting that we all need to find ways to move that bring us a different kind of happiness than what we find in the gym.
Train smart, play often, and you'll discover there is more joy in movement than just a good sweat session.