The official Cuban reason was because of formal treaties, trade pacts, and business interest in and with the United States.
It is interesting to remember that WWI started as a relativly minor "disagreement" between Austria-Hungary and small "unimportant" Serbia, but due to treaties became a war involving all the major powers and most other countries of the world.
The Cuban government stressed a "moral obligation" to enter the war one day after the US did, since it was the US that was responsible for Cuba's independence. They also claimed a shared "moral outrage" against Germany's submarine warfare. Probably more important were practical financial reasons. If Cuba had remained neutral, all Cuban ports would have been closed to any nation at war. Most of Cuba's trade was with the US. There was also a great deal of pressure from US government ( the US felt some Cuban products were vital to the war effort)and US business interests. Remember, Cuba had been independent less than 20 years and US government and US business influence inside and on Cuban government, population and business was very strong.