Kimmel kicked off "Jimmy Kimmel Live" on Tuesday by heaping his affection on the veteran talk show host.
"If Dave had any idea of how excited I am that he’s here with us tonight, he would not, under any circumstances, be here with us tonight," he joked.
Once on stage, Letterman, replete with a flowing silver beard, spoke about what his life has been like since he exited the late night scene in May 2015.
"You're looking at a man who is laughing on the outside, crying on the inside," Letterman said. "I have been looking high and low. I am determined to find a shirt that looks good untucked. I can't find one."
At one point, Kimmel appeared to tug on the legendary comedian’s heart strings, when he asked him if he had become a different person.
Letterman responded, saying, "Each and every day, if you take a look at the horizon of humanity [and say], 'My God, is there anything we can do, big or small, to make the life of one person a little bit better?' [then] that’s no small accomplishment."
Letterman, 70, has mainly stayed out of the spotlight since he left his post at CBS’s “Late Show,” which he hosted for more than two decades, in the spring of 2015.