2001 Honda Civic the battery keeps dying

In December I left my light on for a couple hours, the next morning, of course, the battery died out. About 3 months ago, I tried to turn on my engine, and the battery died again. Yesterday, I parked my car at the library for one hour. When I was done, I went to my car to turn on the engine. The battery died out once more. There was still battery fluid, but it wouldn't start. I had to wait for my friend to come to give my car some juice. I took my car to the repair shop today, and he told me nothing was wrong with my car or battery. He said that I must have left my light on, but I most certainly did not leave my light on. I do not understand why my car keeps dying. The repair man said that there was nothing wrong.