Timothy B. Schmit – who describes "I Can't Tell You Why" as "loosely based on my own experiences" – composed the nucleus of "I Can't Tell You Why" which he then presented to Glenn Frey and Don Henley: Schmit – "I had some writing sessions with Don and Glenn and I threw out a bunch of my ideas and that one [for "I Can't Tell You Why"] stuck. I had [composed] a pretty good part of it, not a huge part but enough for them to think 'That could be good' and go with it. So Don, Glenn and I finished it over a few all night sessions." "When it was being developed in the studio...I knew it was a great song. I [thought] 'Yes! This is an amazing debut for me.' When we finally mixed it, we had a little listening party at the studio. As people were hearing it, Don turned to me and said, 'There's your first hit.'" Schmit himself played bass on the track but believes Frey devised the distinctive bass riff. (Schmit – "I would guess that might have been a Glenn thing, but I don't really remember.")
In 1980 the band promoted the song with a music video which featured Schmit on bass guitar accompanied by Frey on the electric piano, although Frey recorded the guitar solos as it is printed on the LP inside sleeve (Joe Walsh played all the keyboard parts on the actual studio recording), with Henley on the drums, Don Felder on the electric guitar, Joe Walsh on the organ and Walsh's touring sideman Joe Vitale on synthesizer. Live versions of the song were released in the 1980 album Eagles Live and 1994's Hell Freezes Over.