There are all sort of reasons why the heart rate can be faster than normal caused by factors outside the heart as the previous poster stated. Even stomach issues can aggravate the vagus nerve causing the heart to act up a bit. Depending on the speed there can also be structural reasons as well. How fast of a rate are we speaking of? Sinus tachycardia is generally caused by outside influence already discussed but there are conditions where there are extra muscle fibers in the heart that can cause the heart to get caught in a fast beat. These usually start and stop within one beat time and can last from a minute or two or can last hours. The rate is usually over 200bpm and for me felt very manic. This is called an svt/supraventricular tachycardia. It was quite obvious with the one beat start and stop that I was having an episode. That said, if you are having a faster than normal heart rate you should probably see a doctor to find out the cause whether or not there is a structural issue or if it is some outside influence causing the higher than normal beat. All of which can and should be addressed to alleviate the fast rate.
There are any number of reasons. Remember, the object of a heartbeat is to pump oxygenated blood. There are two sensors in the carotid arteries that monitor oxygen concentration (as well as carbon dioxide). If there is anemia, the heart will have to pump at a higher rate to maintain the same unit volume of oxygen. The body can't simply breathe faster (which would increase oxygen) but blow off too much carbon dioxide, disturbing the acid-base situation in the bloodstream. Shock could be another reason. Or hypovolemia and dehydration. In some patients the atrial pacemaker becomes a bit fast because of something as simple as a magnesium deficiency.