Distress signals are irresistible. They mean someone is in trouble, and where there’s trouble is exactly where the Doctor wants to be.
“Are you sure this is a good idea?” Donna’s being the voice of caution again; someone has to.
“I know, but… shouldn’t we go in the back way or something, sneak around a bit, try to find out what’s going on first?”
“We could, but if we do that, by the time we get there we might be too late. This way we’ll have the element of surprise. Allons y!”
“You’d better be right about this!”