that's why even dissertations that you love become very difficult.
Yes, we can enjoy learning even when it's hard, and so we may not realize it was difficult ... but it's like exercise. I love to go running ... but it's still work. It HAS to be work for me to get better. If I just lightly job and never push myself, I'll never get better. Learning is the same way.
Now, I'm not saying it is literally impossible to "learn easy" ... sometimes, perhaps. But it's a core principle of cognition research that learning is more effective if it stretches us. That's how we build new schemas and mental models...by pushing ourselves.