"To be honest, I'm happiest when I'm working – the healthiest," she said Tuesday night on The Late Show with David Letterman. (Watch the full interview below.)
"I think this is an opportunity for me to focus on what I love in life. And I don't think it's a bad thing. I think it's a blessing."
Lohan, 26, escaped a jail sentence last month when she reached a plea agreement in a lying-to-police case that calls for three months in a locked-down rehab facility, starting May 2.
Appearing on Letterman to promote the film Scary Movie V and the TV show Anger Management, Lohan got a warm reception from the audience despite a couple of awkward moments when she bristled at Letterman's questions or simply refused to answer them.
When Letterman asked, "How many times have you been in rehab?" The obviously uncomfortable Lohan answered only, "Several."
"And how will this time be different?" he continued. "What are they rehabbing, first of all? What is on their list? What are they going to work on when you walk through the door?"
"We didn't discuss this in the pre-interview," she answered.
But Letterman continued to (lightly) grill her, asking when she was supposed to go to rehab, and who would be taking her.
Looking away, Lohan awkwardly answered, "Jimmy Fallon … and you. And Tina Fey and Lorne Michaels."
But the actress was quick to clarify, "I don't want people to think I'm making a joke about [rehab]. It's not a joking matter."
Seemingly serious for a moment, Letterman praised Lohan, telling her, "We never thought we'd see you again, honestly, because of the jokes and stuff. But yet you have enough spine, enough sense of yourself, enough poise, to come out here and talk to me."
His reaction caused Lohan to briefly tear up – but the moment of seriousness was punctuated with yet another (awkward) joke, as she reached for a mug on his desk and sipped, saying, "Oh, I thought this was going to be vodka. Shucks!"
"You gotta stop drinking," Letterman jokingly chided. "They don't let you drink in rehab."