(Should): it is used for giving advise to someone.
Example: 1) you should go to party. 2) They should get up early in the morning.
(Must): it is used to pressure someone to do that work.
Example: 1) you must go to hospital. Note:- (to go hospital it is so important for u. Someone press you to go to hospital.
2) you must respect your parents. Note:-( because to give respect your parents, it is so important for you.
Overall:- should is used only for whether you do or not. There is no negative points effect on you.
But must is used for important work that you have to do somehow. Otherwise there is negative points effect on you not to do that work.
Hope you all understood
Thank you..........