Stress most likely - you have other things on your mind. Try meditation, the art of letting go of your thoughts. Practice it a few times a week - probably for no more than 5 or 10 minutes at a time to begin with. The idea is to sit in a comfortable place in a comfortable way and simply not grab on to any of the thoughts that swim through your head. Every thought has a peak and a trough - like waves on the ocean - the idea is to no engage the wave and rise to the peak, but rather to settle into the trough where there is calm. Then just lets the thoughts wash over you without touching them. Eventually you'll make a mistake and get attached to some thought or another; once you realize it, let go of the thought. This can be pretty frustrating, but stay persistent and you'll see the frequency between thoughts expand so that you have a lot less on your mind. You'll begin to "get" it when you realize that you've become much more deeply and intimately aware of what is going on around you. Try this not just while sitting, but also while walking. If you can do it while walking, you may be able to do it during sex. If you can, you'll likely be more aware of the act itself, which can be quite pleasant.