because sometimes you're too drunk. or halfway through you look down and think "this bitch is ugly" or you get hit with some foul odor that makes you just forget about being in the mood to fuck, or your neighbor walks in on you. sometimes the phone rings and its an important call youve been expecting, maybe she made you wear a condomn. possibly the girl you are copulating with has a habit of pinching your ass, or screaming at you to call her "grandma" or insists on referring to you as "bitch" the whole time. maybe you just had a long day and really dont give a fuck. the girl youre with might get sore, or get really sensitive and ticklish after she comes and has to take a break. maybe poop is involved, or it starts to rain and you left your sunroof open.
or sometimes you just dont cum. shit happens man. but life goes on. "la macchina va dove vanno gli ochhi"