Why the sap will compile the abap program again and again?

Every one tell me that when you open some screen at the first time, the sap will compile the abap program and will not compile again at the next time.But I found actually when I open some screen at the 2th or 3th or more times,the sap will still compile the abap again.Such as open the T-CODE va01,mm01,and so on,the sap will show some message below the screen to show it was compiling the abap program.This make the operation spend me huge time.Is there any one could tell me how to resolve the problem or give me some advice?And I will appreciate him/her very much.Thank you.