What drives you? Is there an endeavor you're involved in that requires you to exert yourself mentally, physically, or emotionally? Or even all three? If your answer is yes, then something is driving you to continue because what you're doing isn't easy. That something that is driving you is your purpose. Without a purpose, it is almost impossible to exert yourself in a significant way.
the reason for which something is done or created or for which something exists.
"The purpose of the meeting is to appoint a trustee" · [more]
motive · motivation · grounds · cause · impetus · occasion · reason · point · [more]
have as one's intention or objective.
The overarching purpose of training is to cause adaptation(s)d for anyone who works out. Once this is understood, you need to discover or decide your own purpose. Driving adaptation(s) is the purpose of training, but what is your purpose for causing these adaptations? This specific purpose will determine many of your training program's variables (what exercises you will use, how many sets/reps you will do, what intensity(-es) you will use, etc.). Still, just as importantly, it will drive you to follow that program and keep you working in the gym.
For you to be able to produce more force, your cardiovascular system must improve its efficiency to get nutrients to your muscles better; your metabolism must improve to get the energy to your muscles better, your motor cortex and the neuromuscular system must improve integration to use the muscles more efficiently, your endocrine system must improve to ensure recovery from the work, and on and on. So now you know that to be healthier, you need to be stronger, and you're slowly zeroing in on your purpose.
So why do you really want to be strong? If you're an older person with a family, it might be to really enjoy your time with your grandchildren and/or keep up on vacation when the family is on an adventure. If you're an athlete, it might be to be more effective in competition. If you are a busy professional, it might be to have the energy to power through every day efficiently. If you are a firefighter or EMT professional, it might even be the fact that other people's lives depend on your ability to perform. The stronger you are, the safer other people around you will be. All of these are examples of specific purposes that can really drive someone.