It's old age!
No, I'm not joking; males are at their sexual peak around the age of 14 or 15. From there on (even before it's legal to have sex with a girl of the same age) it's all downhill ;-(
Additionally, such factors as (even very mild) depression can reduce 'libido', as can stress and tiredness from work or relationships.
Somewhat confusingly, things can work the other way round too. People who're depressed or under stress sometimes feel a 'need' (rather than a 'desire') for sexual activity but 'getting things back to normal' in their life can eliminate such a need.
Chris