The Doctor loved solving problems and putting things right. He was at his happiest when trying to unravel a particularly complicated mystery. He had a brilliant mind and delighted in showing off how clever he was at every opportunity. Modesty wasn’t his strong point; what was the point of being a genius if he didn’t use his intellect to help people?
He didn’t bother with minor mysteries though; anyone could get to the bottom of those. He set his sights considerably higher, but whatever he turned his mind to solving, one thing was always certain; it wouldn’t be a cakewalk!