Too much stress, not enough time. Work, busy schedule, failed relationships, summer heat, and a thousand other reasons can get you in this mindset: at the moment, you’re just not cut out for the full time relationship. 
Unfortunately, this doesn’t turn off the endless faucet of sexual desire. So what’s a civilized person to do?
Just as common as the ‘It’s not you, it’s me’ line that gets dropped when ending a fling, one of the biggest cliché’s, yet still used lines, is the ‘I’m not ready for a relationship’.
While this isn’t a CSI case that will require a team of detectives to figure out the true meaning, it does throw the person on the receiving end a curveball that they can’t always hit due to the feelings involved.
Inadvertently, the question always remains, “when will you be ready for a relationship” and more importantly, “will you be ready for a relationship with me?”