"I Can't Tell You Why" is a song by the American rock band Eagles which appeared on their 1979 album The Long Run. The song was written by band members Timothy B. Schmit, Glenn Frey, and Don Henley. Recorded in March 1978, it was the first song finished for the album and the first Eagles song to feature Schmit on lead vocals. The studio version became a Billboard Top 10 hit in April 1980, reaching number 8 on the Billboard Hot 100 and number 3 on the Adult Contemporary chart. It is their last Top Ten hit on the Hot 100. The original key of the song is D major.

Timothy B. Schmit came up with the song title and composed the nucleus of "I Can't Tell You Why", which he then presented to Glenn Frey and Don Henley and they completed the song together. Henley described the finished song as "straight Al Green", and that Frey, an R&B fan as he came from Detroit and grew up with the music, was responsible for the R&B feel of the song. Frey said to Schmit: "You could sing like Smokey Robinson. Let’s not do a Richie Furay, Poco-sounding song. Let’s do an R&B song."

Schmit describes the song as "loosely based on my own experiences". Schmit said: "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 sang the lead vocals on the song, with Frey and Henley singing counterpoint. Schmit also played the bass on the track, which has the distinctive bass riff believed by Schmit to have been devised by Frey. According to Henley, Frey came up with the counterpart on the song, and played the guitar solo on the song.

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