don't eat it or else

Gen 2:17 but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die.

Adam had no clue as to what it meant to die. In this, he had to completely trust whatever dying was. It was not a good thing, and to not eat of this particular tree was the right/good/better choice solely because God said so. He had no proof. He had to follow this directive based only on trust. 

He hadn't experienced or seen death. He didn't know what it was. He had no evil (death) to contrast with the good (life) he had already and had always taken part in. He only knew good, not good, and evil.

Simply stated, if he had fully trusted God, he would not have eaten. Him not eating would have been an act of trust in the Father's warning, that God was telling Adam what was in his best interest. That God only wanted life for him, not death. 

Adam did not believe God but rejected His advance. He decided - and believed - he could do for himself what God couldn't. He decided he could and would be his own god. Man to this day continues on this very same path that leads to death, not life. 

For a further discussion on knowing good and evil, click here. 

For a further discussion on God using evil for good click here.

For a further discussion on how both evil and good reveal something about God click here.