It's almost impossible to be content with your own life when people around you seem so goddamn happy for no apparent reason. It's as if they had some secret that you weren't let in on. Of course, this is a minor annoyance in comparison to the real reason you will never be content. You are genetically designed to be miserable. Think about it.
Evolution favors animals that reproduce the most.
Now, outright horniness is a great motivational factor to mate, but in the end, no one is going to mix gametes with you just 'cause you have a stiffie. Potential partners want to see the goods, they want to see you achieve, they want to see you make lots of cash and score touchdowns and buy them big houses and make them have multiple orgasms. So you're going to have to achieve a lot more than an erection before your genes are gonna be selected. The greatest motivational force for achievement in our society is our lack of contentedness. If you're bored, or upset, or generally not happy with your quality of life, you're more likely to try and make something of yourself.