Most of us would understand that when we're really not feeling hot, we don't get very hot either.
Interestingly, sex is tied to happiness in a surprising number of ways. It's hard to say if not having sex makes someone unhappy, but we can point to several studies that indicate having more sex leads to a more positive outlook [source: Alexander]. There was even a 2002 SUNY Albany study that showed that women who had unprotected sex had lower levels of depression than women who used condoms or abstained from sex [source: Abbasi].
A University of Colorado Boulder professor also released a paper that used national survey data to find that those who had a higher frequency of sexual encounters reported a happier outlook in general [source: University of Colorado Boulder]. He also saw that no matter how often people reported having sex, if they thought they were having sex less than their peers, they reported lower happiness.
So not only a lack of sex drive, but also a perceived lack of sex might be a trigger that you're not doing so hot, emotionally.