Why should a rebellion in Cuba (an internal Spanish issue) become a cause for war inthe US?

There are at least three reasons for
this.

First, because of its history, the US tended to be
sympathetic to groups that were rebelling against a colonial government.  Many Americans
thought that a country that had itself rebelled against a European monarchy should help
rebels who were doing the same.  In this sense, the US saw the rebellion as a cause for
war due to ideological reasons.

Second, there were those in
the US who were looking for any excuse to go to war.  These people, like Theodore
Roosevelt, believed that the US was growing soft.  They believed that the nation needed
to have a war to make Americans tougher.   This group saw Cuba as a convenient way to
test and enhance American masculinity.

Finally, there were
those who wanted the US to have an empire.  This motive was mixed up with the previous
motive.  These imperialists felt that the US needed to keep up with the European
countries that all had and/or were gaining empires.

For
these reasons, an issue that was seemingly internal to Spain became a cause for war in
the US.