I listened to a podcast on the Avatamsaka Sutra recently. In the talk, the speaker commented that the Avatamsaka sutra was a critique on the arahants. He argued that while the arahants were saints in their own right, they were inferior to the Bodhisattavas because the Bodhisattavas were motivated by the compassion to save all beings while the arahants focus only on their own enlightenment.
I have heard similar views being expressed many times by various Mahayanists. In fact, these views generally fall into 2 categories:
“Strong” version: Mahayanists in this camp claim that Theravada (and the arahants) are selfish and only care only about their own enlightenment and not much else.
I think very few Mahayanists hold this view these days. Anyone who has met a well practicing Theravadin Buddhist will know that this is not true. This view is also not supported by Theravada doctrines (both the Nikaya and the commentaries), nor by history. In fact, if Theravada Buddhism is truly selfish, it would have died out within one to two generations after the Buddha’s passing because everyone would be too selfish to teach it to others.
“Weak” version: In the “weak” version, Theravadins are held in higher regard. Mahayanists in this camp believe that Theravadins want to reach their own enlightenment, before going out to help others.
I think this is too simplistic and is not in accordance with reality; I personally know many Theravada Buddhists who are dedicated volunteers and Dhamma workers, much like their Mahayana counterparts.
Compassion is a common human quality and we run against reality when we try to put Buddhists neatly into 2 camps; One with lots of compassion and the other with little. Reality is usually more complicated than that.
 Compassion is not an idea exclusive to Mahayana Buddhism. It is not even unique to humans only. There are many reports of animals showing kindness and compassion to each other and even to humans.
 So what differentiates Mahayana from Theravada Buddhism? Well, its a long story! But I would say that the core differences comes primarily from where they draw their teachings. Theravada Buddhism is based on the teachings found in the Pali Canon (the suttas, Abhidharma and the commentaries) while Mahayana is based on the Mahayana sutras, Mahayana commentaries and the teachings of Mahayana teachers.
Therefore the goal of Theravada Buddhism is Arahatship and the suttas themselves were silent on how one can become a Buddha. While in Mahayana Buddhism, the Bodhisattava ideal (and the Bodhisattava vow to save all sentient beings) is held in highest regard.
This may explain why Mahayanist sees Theravada Buddhism as “selfish”. However, aspiration aside, on the ground level I know many Theravadins who are equally committed to serving as any Mahayanist.