Actually, the highest seat of Mahayana teachings, which is the Lotus Sutra, explains...
Life is fundamentally Buddha. That means that in the remote existences millions of kalpas ago, we have all attain enlightenment. What happened is the accumulation of negative karma clouding the Buddha nature.
By helping others to "realise" their own innate enlightenment, it is to rekindle our own. It is said that the purpose for the existence of Shakyamuni lies in his behaviour as a human. That means that Shakyamuni's task is to aid all to mankind to attain the same enlightenment as himself, urging others to attain the dharma and Buddha body just as he did.
Shakyamuni himself is highly community-orientated resulting in the formation of the Buddhist Order.
In his earlier teachings, he taught the three vehicles for attaining enlightenment which are "learning, realisation and bodhisattva". Theravada Buddhism focus on learning and realisation. Mahayana focus on bodhisattva.
However, in the parable of the Great White Ox, it is stated that the three vehicles were all expedient means and that there is truly only one Buddha vehicle.
Neither the Theravada "inward-looking" or the Mahayana "outward-looking" is complete. One of the key idea of Buddhism is actually to recognise the oneness of man and environment. That the two is intrinsically related and all life are manifestation of the Buddha life.