"I can't use the excuse of being alone anymore. I won't survive a battle just because I'm young. I'll need strength!"
Lancaster stood firm on his decision of trying to get stronger, even though Lilith played his concerns down.
Castiel's words had made a deep impression on him, especially when he talked about Lancaster being far away from his true potential. From the way Castiel talked, it appeared as if something big was about to come... Something worse for them, for which they needed to be prepared.
If Castiel had thought the same thing as Lilith, he wouldn't have sown a seed of doubt in his mind.
When it came to time, he trusted Castiel more than Lilith for some reason.
"Can you tell me how I can achieve it?" He asked clearly, once again. "Forget about the time not being right and the dangers. Just help me achieve it. Help me become so strong that my father would've been proud of me."