Sex requires moisture. Lubrication is one of the most important ways to ensure a pleasurable sexual encounter. Sometimes your body produces enough to last you through sexual endeavors, and sometimes it doesn't. The amount of your natural lubrication can depend on where you are in your menstrual cycle, your age, or even your stress level. And condoms can soak up natural juices quickly.
Using lube makes any kind of sex more comfortable, more fun and more spontaneous. Look for Silicone Lubricants if you want a long-lasting experience or water-based lubes to wash off easily. Add a sprinkle of water to your water-based lube if it starts to dry out.