Here is the full sentence I have found:
Does it mean 'me'?
Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It only takes a minute to sign up.Sign up to join this community
As @Eric says,
あれほど means "so much" or "to such an extent". Best I can tell, this sentence says, "Given that you used to hate boxing so much, why have you suddenly become so seriously motivated [to box]?"
However, as @oldergod mentions,
あれほど would mean close "to such an extent", or "that much" rather than "so much".
You hated boxing
that much, what suddenly made you become so motivated [to box]?"