The most important reason for this was military. It was felt that annexing Hawaii would make the United States stronger.
Annexing Hawaii would make the US stronger in two ways. First, it would give the US a naval base out in the middle of the Pacific. At that time, it was felt that having a big navy with lots of bases was the way to be a major power in the world. Second, it would prevent any other powerful nation from claiming Hawaii. If another powerful country owned Hawaii, it could use it as its own base and potentially threaten the West Coast of the US.
Please note that sugar was not a real reason for annexation. America did not need Hawaiian sugar and annexation was more of a boon to the sugar growers in Hawaii (who were of American descent) than to the US.
The Hawaiian Islands were considered a stepping stone to the Asian markets that the entire world was trying to develop, especially China. The United States understood that the Chinese market meant over one billion potential customers for America's growing industrial capacity. Chinese demand would hopefully end the boom and bust cycles of American business. Also, Alfred Thayer Mahan's History of Seapower was a very popular book in the 1890s--the book's premise was that any strong nation needed a strong navy in order to protect its shipping. Mahan looked at the Dutch and English navy, and he said that America needed a powerful navy too. In order to fuel this navy, one needed coaling stations all over the world. Hawaii had excellent harbors and was halfway between America and China--it would be perfect.
Of course, there was also a matter of sugar and pineapple plantation owners in the islands who wanted to sell their goods in America duty-free. These plantation owners (one of whom was the founder of the Dole Pineapple company) overthrew the Queen and petitioned to join the United States. Grover Cleveland, America's president at the time, saw this as an unlawful land grab, so he refused. The president after Cleveland, McKinley, accepted the annexation of Hawaii on the grounds that it would prevent the islands from falling into British hands.