"I Can't Tell You Why" is a song by the American rock band Eagles, which appeared on their 1979 album The Long Run. It 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. Released as a single in February 1980, it became a Billboard top 10 hit in April of that year, reaching number 8 on the Billboard Hot 100 and number 3 on the Adult Contemporary chart. It was the group's last top ten hit on the Billboard Hot 100.

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."