"Can't really test that with science," Bruce pointed out. "The Hulk defies all scientific and logical explanation. And the only way to know beyond all reasonable doubt what would happen is to actually let it happen." And he still wasn't sold on that idea, especially as he knew the kind of destruction and devastation the Hulk could and had wrought on Earth.
Animals were soothing creatures and scientifically proven to reduce stress so the more time spent around them the better people started to feel.
"Just given how long he was in control last time makes me think that if he gets a chance to get back in the driver's seat then I don't know how I would break through."