Turgon's Folly

[Maedhros]

Why did Turgon became proud and refused the aid of Ulmo?

Then Turgon pondered long the counsel of Ulmo, and there came into his mind the words that were spoken to him in Vinyamar: 'Love not too well the work of thy hands and the devices of thy heart; and remember that the true hope of the Noldor lieth in the West, and cometh from the Sea.' But Turgon was become proud, and Gondolin as beautiful as a memory of Elven Tirion, and he trusted still in its secret and impregnable strength, though even a Vala should gainsay it

Was it because he was loath to abandon anything he had set his mind to do?

Moreover Fingon and Turgon were bold and fiery of heart, and loath to abandon any task to which they had put their hands until the bitter end, if bitter it must be.

Why is it that he trusted Turgon in the beginning and not in the end?

It has been told how by the guidance of Ulmo Turgon of Nevrast discovered the hidden vale of Tumladen; and that (as was after known) lay east of the upper waters of Sirion, in a ring of mountains tall and sheer, and no living thing came there save the eagles of Thorondor. But there was a deep way under the mountains delved in the darkness of the world by waters that flowed out to join the streams of Sirion; and this way Turgon found, and so came to the green plain amid the mountains, and saw the island-hill that stood there of hard smooth stone; for the vale had been a great lake in ancient days. Then Turgon knew that he had found the place of his desire, and he resolved to build there a fair city, a memorial of Tirion upon Túna; but he returned to Nevrast, and remained there in peace, though he pondered ever in his thought how he should accomplish his design.

What happened to him? Why did he lost faith in Ulmo?
All quotes are from the Published Silmarillion.