Despite his initial and quite adamant refusal to board the newly completed Thousand Sunny - which resulted in that extensive and somewhat humiliating chase through the streets of Water Seven in pursuit of his sea panties - he was greatly relieved when Luffy insisted that he join the crew. Because while the simple act of building the ship did indeed fulfill something of his dreams, he wasn't entirely truthful regarding the full extent of them. In reality, he wanted to go with them from the moment he asked them to sail the ship that had until now existed only on blueprint parchment, even before he took up his tools and the first plank of Adam wood.
He's always secretly contemplated his shipyard creations as his children, and seeing the fruits of his labor turned against him and used to rain down death and destruction on innocent people during his days as Tom's apprentice- well, it crushed something deep inside him that's only begun to bloom anew as he watches his new nakama explore the ship from bow to stern, chattering excitedly over each room and each new surprise as it's revealed.
After witnessing their tearful farewells to the Going Merry, he knew he'd be leaving his masterpiece in excellent hands even if he bid the Straw Hats farewell and left them to their own devices. They're determined and resilient people; he knows because he watched their actions quite closely during their assault on Enies Lobby to rescue Nico Robin- who's also proved herself a strong and resourceful woman. With an impressive grip.
But there are disturbing parallels that he can't help noticing: two ships built of Adam wood, two "D" pirate captains and two crews loyal unto death...
He considers the Oro Jackson's plight - captain executed, crew disbanded, the whereabouts of the vessel itself unknown - and it strengthens his resolve. He'll serve as shipwright, maintain the Thousand Sunny and do everything in his power to keep his sea-going child and her crew safe.
History will not repeat itself.