, the simple reason is copyright. As you don’t appreciate your writing plagiarized by others without permission for personal or commercial use, neither does a music artist.
Some writers think they can get away with incorporating lyrics into their novel without being caught because after all, it’s only a couple of lines. But in today’s world of advanced technology and quick, easy access by millions of users, rest assured you will get caught. When you do get caught running afoul of trademark law, those two lines you embedded within your novel will not be worth the distress it will cause.
Being in violation of trademark law may require you to pay fines, destruction of all unsold copies of your book, payment of royalties for all published work sold to date, and much more.
As a writer, you should respect others artistry. Also as a writer, you should be capable of writing your own lyrics or incorporating mood setting music in your book in alternative ways. For example, if you want to combine a song by the Bee Gees into your writing you may want to write, the falsetto voices of the Bee Gees came from the radio singing to love somebody or Madonna crooning slyly about being like a virgin drifted through the radio.
There’s no law against printing a song’s title or referencing the song in your writing. Just don’t quote the actual lyrics to any song unless you have permission from the artist to do so.
If you absolutely must quote lyrics in your book, use lyrics from songs written before 1923. In the United States, much of the music published before then is in the public domain, meaning it’s free for anyone to use for any purpose.
Regardless, caution should be exercised when considering the use of someone else’s work. Check whether it’s copyrighted or not to avoid running into legal issues. If you recall, “Happy Birthday” until 2013 when the lawsuit against Warner/Chappell the publishers who claimed to own the copyright to the song was launched and was widely publicized, many considered the song to be in the public domain. Now, not so much.
Your best option is originality. Write it yourself.