control system positive phase margin ,gain margin physical meaning and intution

[tronicsworld]

i know for stability we need positive PM GM . that means at -180 degree gain should be negative(in decibels) for stable systems . From where this arrives i mean why -180 ?? i read somewhere that after -180 degree NEGATIVE FEEDBACK BECOMES POSITIVE FEEDBACK and if at that time gain >1 than system will oscillate and become unstable ?? why negative becomes postive feedback . I can understand this for a sine wave where after 180 degree it becomes 0 again but what for circuits where phase lag greater than 180 degree is possible ???