One thing the Doctor loved about humans was their curiosity. While some were set in their ways, uninterested in anything beyond what was familiar, others seemed to spend their lives in a constant quest to experience new things, sometimes going to extraordinary lengths to do so.
Because of their adventurous nature, the Doctor favoured humans as companions over most other races. He occasionally had non-human companions, variety was good, but more often than not he’d find an exceptional human on one of his visits to earth and talk them into taking a trip with him.
One trip was seldom enough.