Here is a quick and dirty reason why: You could cause your body to stop producing testosterone during the time when it is producing the most it will ever produce.
Longer version:
Assume your natural T level is 1200 right now. You want massive gains, so you take 300mg of Test-Cyp a week, which raises your T level by around another 1200...turning you into a monstrous 2400!!! Yes, massive gains, but also massive estradiol levels...but those can be taken care of with an AI. So far, so good. However, when outside T is introduced into the body, it stops making its own T (it is expensive to make, from a biological standpoint) and if the PCT is not done right, your body shuts down its own T creation forever. Now look what you did...the shots you are taking merely raise you up to the level you were naturally! To get that massive 2400 again, you have to double the dose!
So you have a good chance of destroying all the natural awesomeness your body has all by itself for short term gains. Instead, spend the time and money getting your diet in shape. 3JS (on this forum) will help you do that for a fraction of the cost of a cycle. Once that is in line and you are exercising hard, you will see the massive changes you want. When you get older and your natural test drops, then think about AAS.
