Again, I'm in the camp that believes we aren't exactly sure why so many men find themselves in the low T category. Modern stress, poor diet, environmental factors (some known and likely many unknown), obesity etc.
Sure, fix those and you will likely improve your overall health including T levels. But, will this be a tangible benefit? Yes, it will be, but I'm not sure this is going to significantly improve your T. Overall, you'll be much better off, but it's likely your Testosterone production won't ramp up to anywhere close to optimal.
I believe a lot of guys experience a decline in T which PRECIPITATES other declines in health. Behaviors change. Under appreciating this is a mistake. So, at some point you have to stop worrying about the "why". There are plenty of docs out there willing to help you optimize your hormonal milleu.
Optimize the T, and make other behavioral changes discussed above. But, making those changes will be easier with an optimal T. Please realize this.