what does Confucius mean when he said Is it not pleasant to learn with a constant perseverance and application?
Confucius is recognising the necessary role of perseverance and application in the learning process. To truly see and experience the transfomational benefits of learning, we have to be serious and diligent about our learning and approach it in a structured and industrious matter. We can't expect to gain any benefits from our learning if we merely play at it and do not take it seriously.
I think that what Confucius is saying is a more refined way of saying "no pain, no gain." He is saying that the things that are truly rewarding in life are the things that you have to work for. Constant effort is not pleasant, but the end result is worth it.
It might be worthwhile to move this question to the Literature Discussion Forum so you could get other viewpoints in response to this quote.
My interpretation would be that Confucius is recognizing the value and fulfillment that comes from having to apply some significant effort in order to learn something. Most students would agree (sometimes reluctantly, sometimes only to themselves) that they value more highly knowledge that is gained after engaging in a real struggle to master the topic than information that is quickly and easily assimilated. It's more rewarding to conquer a difficult subject than to slide through an easy class. Confucius is recognizing that it is pleasant to exert oneself in order to learn.
Check the original text, you will find out what Confucius means:
Learning (new things) is enjoyble. is it more enjoyble to go over it regularly?
It is good if you have friends around sharing knowledge, is it not also a pleasure sharing knowledge with friends from non-local visitors?
You feel good if someone understand/agree with you, will you feel better if you can tolerant those who don't understand/agree with you?