Luiz's £34million move from Paris St Germain to the club he first joined in 2011 was the biggest surprise of the summer with the erratic Brazil international reinforcing the Blues' resources at centre-back.
"When we take the decision about the players, we always take the decision together - me and the club," Conte said.
"I told you that this transfer market was very difficult for us to reach our targets, but at the last minute, on the last day, there was this situation which was good for me and the club to improve the squad.
"For this reason, we bought David Luiz. We invested in David, and I'm very happy. We took a player with a great personality, good technique and who likes to play.
"I like to play football and I like defenders who like to play football. We know that David can improve in defensive situations, but it's good and I'm very happy he has come here.
"He's a good guy, a positive guy for the changing room. I'm very pleased for this."
v The 29-year-old is a gifted player but is also unpredictable and has been prone to making risky decisions - shortcomings Conte intends to address.
"I think every player, until the end of their career, can improve," said Conte, who indicated Luiz is unlikely to start Sunday's Premier League clash with Swansea as he adapts to Chelsea's new "philosophy".
"Even when you are 32, 33 or 36 you can always improve. I tell you this from my experience as a footballer.
"David Luiz is 29 - a fantastic age for him to become one of the best defenders in the world.
"I'm seeing great commitment from him. He wants to play for Chelsea. This is a great opportunity for him - and for us - to improve.
"I think that the coach must improve his players. I think that's the most important thing for me - not only to win, lose or draw.
"To improve your players and to work. To work on weaknesses and make the players stronger."