Why did the US become allies with England in World War 1?

"England isn't a country. We sided with Great Britain."

Wrong again.

Great Britain is not a country either.

The country at the time which allied with the US, was the UK; United Kingdom of Great Britain and [at the time not northern] Ireland.

It is like saying "mainland america", it is a geographical term, more commonly mistaken for "the island of Britain", Great Britain being the outlying islands. Only Americans call it Great Britain. NO ONE in the UK calls it Great Britain apart from an atlas which includes all the islands around it.