This relationship started at the end of Queen of the Damned when David was still an old man working for the Talamasca. Lestat fell in love with David, as he often will, because of David’s refusal of he Dark Gift. He seems to find this virtue in mortals fascinating or impressive. They become good friends in the years that follow Queen of the Damned. David was the only one who helped Lestat when he was being screwed over by the Body Thief since Louis and Marius had turned him down. That’s why most of us love David: His love and aid are unconditional.
Lestat cannot live alone it seems and must have believed himself to be on the outs with Louis so he made David. But I’m sure he couldn’t live without David. I think it was inevitable that David was to be made a vampire and since those who ask for it go quicker than those who don’t it was good that Lestat made him against his will. And David doesn’t resent Lestat for it. That’s new.

“I thought you’d found a way to die. You’d given me every drop of blood inside you. And now you yourself were perishing before my very eyes. I knew I loved you. I knew I forgave you. And I knew with every breath I took and with every new color or shape I saw before me that I’d wanted what you’d given me.”

David seems a more suitable companion for Lestat. When Lestat was catatonic after his run-in with the Devil, David cared for him and talked to him. And Lestat is good for David since when Louis and Merrick were together, David was a bit jealous but Lestat reminded him that his maker was there for him.