1. desired to become more internationalistic and imperalistic so they keep their eyes on the European powers who were developing their colonial powers
2. wanted to possess a naval base of operations to see after their possessions at the time , namely the Philippines, Guam etc.as their protectorates
3. Hawaii was an important stop for whaling ships and a growing sugar producer. To ensure the US maintained preferred market access and not France or the UK, they took it.
Hawai'i was never legally annexed to the United States. Annexation involves "cession", which is the transfer of dominium (sovereignty/authority) and domain (real estate). Cession is always a bilateral transaction between two sovereign states, which is achieved through a treaty of cession or treaty of annexation. In other words, it's just like transferring the title of a car from one person to another; a contract needs to be created and both parties need to sign/agree to it in order to transfer a title from one individual to another. There were two attempts to annex the islands by treaty in 1893 and 1897; both failed. The second failed due to political activism for Hawai'i did not want to be incorporated into the U.S. When the Spanish-American War began in 1898, the United States then had a heightened interest in Hawai'i as a military outpost. America then passed a joint resolution claiming to annex the islands. However, an American joint resolution is not a treaty of cession; the joint resolution is a piece of American legislation that is limited by U.S. borders, not legally capable of annexing foreign territory or extinguishing the existence of a foreign sovereign country. And since Hawai'i was never legally annexed, the islands were seized regardless, making them occupied.