batman is right (as superheroes usually are) - your sex drive will go through ups and downs depending upon lots of things - diet, environment, stress, how much sleep you're getting. Weather affects me sometimes even! And it's different for everyone.
It might also be because sometimes you just need something to 'get you started.' This is definetely the case for me because i'm very much a doing person - I become 'motivated' to do things by doing them. I'm the same way with sex. I might not be in the mood at all until a few kisses and a few light touches, and then... boom! I'm horny as all hell

Erotica works in the same way... if you think and act sexually, it works wonders. I can get myself extremely worked up just by visualising a scenario in my head, or sending my partner a dirty txt/email telling them everything I want to do to them when I get home.
I've noticed a few different posts from you in the past couple of days, about a few different topics. It sounds like there's a lot on your mind at the moment, which can be a contributing factor to libido (or lack thereof)