When i begin masturbating i can't obtain an erection until the middle and end of the masturbation. before i could get the erection instantly. why ??

See answer : There are many internal and external factors and variables that can effect the ability to achieve and maintain an adequate erection. In an otherwise healthy adolescent and young adult male, situational, temporal, and psychological factors such as performance-related anxiety would be the most likely cause of your delay in achieving a good erection and should not cause you to be overly concerned.