He knows that an honest answer will likely earn him scorn and derision from Megatron, but anything less than the truth would be horribly inadequate. He's endured plenty of scorn for his beliefs already -- he can take it from Megatron, too.

"A way of healing. Atonement." He won't deny that he has much to atone for. That's what keeps him grounded, humbled. "Being a force of good in the universe, instead of one of chaos or destruction or oppression. Helping others in need. A good friend of mine once told me that helping others is the highest cause you can aspire to." He does look at Megatron then, his expression momentarily unreadable. "But he wasn't the first person to encourage me to help make the world a better place."