Why do muscles grow? The actual biological reason muscles grow is probably better off left for the scientists and doctors.
A good way for us to understand it is simple evolutionary process.
Your body has evolved to survive and protect itself.
Simply put:

• When you haven't eaten, you feel hungry
• When you haven't drank, you feel thirsty
• When something harms you, you feel pain

"Hypertrophy" or muscle growth, can be seen in the same manner.
When you go to a gym and lift weights, you are putting muscles under stress.
Each repetition closer to the point when your muscles can no longer perform creates deep inroads into the fibers of muscle.
This causes micro-tears in the muscles.
You are intentionally threatening your survival.
The body reacts by repairing the muscles bigger and stronger than they were.
This process is intended to protect the body from future threat.
I know this sounds like a very basic idea but it is really that simple.
Micro-tears by weight training > muscle tissue repairing itself = Bigger and stronger.
How Do You Do This? Let me boil this down to a simple process.
There are two little words or laws you need to burn into your brain.
Intensity and Progression.

• Intensity - The amount of effort exerted on each set of each exercise.
  
• Progression - Increasing the repetitions or the amount of weight for each exercise.
  

These two principles are the root of true muscle growth.
In order for muscle to grow your body must perceive training as a threat to survival.
You should train at the highest level of intensity as you safely can.
Then you need to progressively increase the weight you lift or the amount of repetitions to continue the process and to build muscle mass quickly.
When you go to the gym, you are going to war.
You should Get checked out by a doctor so you are sure you can train like this.
You also need a positive mental attitude so you don't lose sight of your goals.
Be sure you have a good solid plan and then take action.
No plan, no action = no results.