Libido or Sex Drive is defined as desire for sexual activity or urge for sex; one may or may not act on this urge. It is different from Erectile Dysfunction in which a man has the urge but is unable to have a sustained strong penile erection to perform the act.
The urge to have sex may be spontaneous, may be due to reflection on memorable sexual moments or visual stimulus(i).
The seat of libido lies in the brain which produces the neurotransmitter called Dopamine; it is produced in the mid-brain. This Dopamine initiates several activities which culminate in the production of testosterone; the male hormone responsible for sex drive.