Summary: The Doctor prefers to do things his own way.
By Time Lord standards, the Doctor is something of a maverick. He goes his own way, does his own thing, doesn’t adhere to the time-honoured traditions… He helped himself to an obsolete TARDIS and set out by himself, without even the usual crew, which might account for the erratic manner in which he flies his time and spaceship. This TARDIS, like all the others, was never designed to be flown by one person.
Going solo freed him of all constraints though, he doesn’t have to answer to anyone, can go wherever and whenever he chooses. Nobody can tie him down.