"Baby I Love U" is a pop and R&B ballad about unconditional love, with critics pointing out that it was inspired by her relationship with the actor Ben Affleck. The song was well received by critics who complimented Lopez vocals. Commercially, the song was as successful as the other singles from the album in the UK, where it peaked at number 3. In the United States, the song peaked at number 72 on the Billboard Hot 100, and reached the top thirty of the Mainstream Top 40 chart.
While recording her then-upcoming album, Lopez listened to blues and soul, the music she had often turned to while growing up. Using songs by Stevie Wonder, Luther Vandross and others as inspiration, she attempted to elicit a similar feeling in her own songwriting, claiming, "Those were just songs that made my heart sing," she said. "They just stay with you. ... I wanted to do something that's true to me and how I grew up, but I also wanted to make beautiful songs. So I'd listen to records from today and I would listen to records from back then, and [eventually] I'd go, 'That's what I want.'"
Among the songs on the album, "Baby I Love U!" was written, with the help of her longtime collaborator and friend Cory Rooney. "He's so talented," she said. "And he's so 'behind the scenes,' especially when it comes to me. He's really, really just dedicated to music for the pure love of music." "Baby I Love U!" was released as the album's fourth and final single on August 3, 2003 in the U.S.., and on March 8, 2003 in the UK.
"Baby I Love U!" performed much lower than Lopez's previous singles from "This Is Me...Then", missing the top-forty on the Billboard Hot 100 chart, debuting and peaking at number 72, on the issue date April 3, 2004. Eventually, the song became Lopez's lowest charting-English single on the Billboard charts, until "Papi" became her lowest charting-single, later in 2011. The song also made its debut on the Hot R&B/Hip-Hop Songs chart at number 75. The song received a Billboard Spin Award on March 25, 2006 for 50,000 spins. Elsewhere, the song proved to be more successful. In the United Kingdom, the song was a huge success, debuting and peaking at number 3, on March 20, 2004, becoming her third top-three single of the same album. In Ireland, the single managed to peak inside the top-twenty, reaching number 16, on March 11, 2004.
The music video for the song was directed by Meiert Avis and premiered on BET, on August 11, 2003. The video was also included on the DVD "The Reel Me". It features scenes of Lopez in the bed, as well as walking around the house, performing the song. Jason Shawhan of About.com called the video "minimalist and stark and much more inspired than the song that forms its spine." Shawhan continued, "Visually, in fact, there's something quite experimental going on with its almost exclusive use of close-up and shifting focal length. This is certainly a promising development in Lopez' continuing evolution as a visual artist, and we can hope she'll work with director Meiert Avis again soon."