The problem was that my Developer Certificate on Keychain was under "System", on the left hand panel, which is always locked, requiring my password to unlock!
Then I moved the Developer Certificate to "login", which is always unlocked, and now everything works perfectly. The annoyance of xCode asking for the password is gone.
Please see this picture for more details:
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…