Why is Willy so proud of his ceiling?

I see it as part of Willie's contradictory nature. He often says two opposite things at the same time. Remember in Act 1, he says that Biff is a lazy bum and then says: one thing about Biff, he's not lazy.

This is beyond confusion; he is constantly conflicted in the things he believes. He is the ultimate salesman who believes his own sales pitch, yet knows it's a sales pitch.

Part of him respects people who work with their hands. He is proud of all the work he has done on his house... the stoop he has made and all the repairs he has done. He says he know every nail. Indeed, he has poignant memories of his father who produced and sold his own hand-made flutes... thus the sound of the flute as the play begins.

Willy is torn and misled by the modern world in which he lives... a world that at once reveres the honest work of one's hands yet pays far better the one who wheels and deals.