A need exists.
His mother thinks that he can take care of the problem.
Although he tells her that it isn't his hour, she assumes that he is going to take charge.
Some commentators think that his initial response to her by addressing her as "woman" is negative; others assert that this address is not rude but more like saying "ma'am" or "madam."
In either case, Mary, the one who has known him all his life, just assumes that he is not only able to solve a problem but is going to.
Sideline: note the event is on the third day, an important day in the Bible.
Another sideline: significance of term "hour": In their commentary on John, Gail R. O'Day and Susan E. Hylen discuss the importance of the term "hour" in this gospel. Although sometimes it does mean hour, that is, what time it is, the noun, hour, is also used as a theological term to denote the eschatological time--last times.
He may have demurred initially, but with this first miracle, his hour has begun