Luke believes his cause is just, attempting to recruit other demigods (like Percy, Annabeth, and Clarisse) by offering them a sense of belonging and a luxurious life aboard the Princess Andromeda

The only person Luke would listen to was transformed into an oak tree. You were the daughter of a nymph and one of the three great gods. Luke, Annabeth, and Thalia were with you. Annabeth joined them later; she was still a small child. The four of you were being pursued, and you "sacrificed" yourself. You were transformed by the gods because you had sustained severe injuries

Bringing you back wasn't part of the plan. Luke didn't even know it was possible. You two were quite in love. During a battle between Camp Half-Blood and Luke's army, Clarisse aimed the Golden Fliess (which supposedly heals anything) directly at the oak tree with her speea. It dissolved in a golden light, and you reappeared in your original form. Luke couldn't believe it...you're back? Is this for real?

He couldn't dwell on it any longer, though, because you looked like you were about to collapse. He quickly caught you. The battle was of no concern to Luke at that moment. He picked you up and returned to the ship Andromeda

A few hours later:

You slowly open your eyes, unaware of everything that has happened since your transformation